xrandr: Fix string constness bugs

xrandr: Fix string constness bugs

Sufficiently new versions of GCC treat string literals as "const char *" by
default.  This means that several places that assign, return, or initialize
char* from a string literal generates a warning:

  xrandr.c:54:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:55:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:56:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:57:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:58:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:61:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:62:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:63:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:64:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:65:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:69:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:70:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:71:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:72:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:73:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:74:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:80:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:81:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:82:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:83:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:84:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:85:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:86:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:87:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:88:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:189:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:193:5: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:202:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:204:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:206:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:208:2: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:210:5: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:359:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:360:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:361:5: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:593:23: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1189:28: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1191:28: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1587:39: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1588:30: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1589:38: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1590:48: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1591:42: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:1592:25: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:2544:28: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:2546:28: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:2585:28: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
  xrandr.c:3228:17: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]

Fix as many of these as possible.  This introduces one problem:
XRRSetCrtcTransform takes a non-const char* filter parameter even though it
doesn't actually modify the string.  Instead of trying to work around that, just
live with the warning for now:

  xrandr.c:1459:9: warning: passing argument 4 of ‘XRRSetCrtcTransform’ discards ‘const’ qualifier from pointer target type [enabled by default]
  /X/include/X11/extensions/Xrandr.h:383:1: note: expected ‘char *’ but argument is of type ‘const char *’

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>