uml: signal handling tidying

This patch tidies the signal handling code slightly.

pending is renamed to signals_pending for symmetry with signals_enabled.

remove_sigstack was unused, so can be deleted.

The value of change_sig was never used, so it is now void and the
return value is not calculated any more.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>