compositor: Return a user-defined exit code

compositor: Return a user-defined exit code

Currently, once we've reached our main's wl_display_run(), we always
return ret=EXIT_SUCCESS when weston terminates through wl_display_terminate.

This patch makes it possible to specify another return value by setting
prior to terminating Weston. This is useful for automated tests that want
to report test failures to the overlying testing infrastructure.

Signed-off-by: Frederic Plourde <frederic.plourde@collabora.co.uk>
[Pekka: fixed some tabs.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>