compat: move strdup(3) replacement to its own file

compat: move strdup(3) replacement to its own file

Move our implementation of strdup(3) out of compat/nedmalloc/ and
allow it to be used independently from USE_NED_ALLOCATOR.  The
original nedmalloc doesn't come with strdup() and doesn't need it.
Only _users_ of nedmalloc need it, which was added when we imported
it to our compat/ hierarchy.

This reduces the difference of our copy of nedmalloc from the
original, making it easier to update, and allows for easier testing
and reusing of our version of strdup().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>