weston: add wait-for-debugger option

weston: add wait-for-debugger option

When you need to start Weston via weston-launch, systemd unit, or any
other runner, it is annoying to try to get in with a debugger,
especially if the thing you are interested in happens at start-up. To
make it easy, a new option is introduced.

The new option, implemented both as a command line option and a
weston.ini option, raises SIGSTOP early in the start-up, before the
weston_compositor has been created. This allows one to attach a debugger
at a known point in execution, and resume execution with SIGCONT.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Ian Ray <ian.ray@ge.com>