help: add "man.viewer" config var to use "woman" or "konqueror"

help: add "man.viewer" config var to use "woman" or "konqueror"

This patch makes it possible to view man pages using other tools
than the "man" program. It also implements support for emacs'
"woman" and konqueror with the man KIO slave to view man pages.

Note that "emacsclient" is used with option "-e" to launch "woman"
on emacs and this works only on versions >= 22.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Tested-by: Xavier Maillard <xma@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>