startx: don't attempt to catch SIGKILL

Most shells ignore attempts to catch SIGKILL. Some, such as NetBSD's
/bin/sh, print an annoying message helpfully reminding you
that it isn't possible to catch SIGKILL whenever X starts.

Signed-off-by: Nia Alarie <nia@NetBSD.org>
6 jobs for startx-sigkill in 1 minute and 44 seconds (queued for 2 seconds)