winsys/sw/xlib: Prevent shared memory segment leakage.

winsys/sw/xlib: Prevent shared memory segment leakage.

Running piglit with this was causing all sort of weird stuff happening
to my desktop (Chromium webpages become blank, Qt Creator flickered,
etc).  I tracked this down to shared memory segment leakage when GL is
not shutdown properly. The segments can be seen running `ipcs` and
looking for nattch==0.

This changes fixes this by calling shmctl(IPC_RMID) soon after creation
(which does not remove the segment immediately, but simply marks it for
removal when no more processes are attached).

This matches src/mesa/drivers/x11/xm_buffer.c behaviour.

v2:
- move shmctl(IPC_RMID) after XShmAttach() for *BSD, per Chris Wilson
- remove stray debug printfs, spotted by Ian Romanick

NOTE: This is a candidate for stable branches.

Reviewed-by: Brian Paul <brianp@vmware.com>