xapp_xinit: revert removal

Even though xinit isn't included in the X11R7.7 release it is very
useful for debugging and quick tests.
Reinstante it without xdm requiring it though.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>