common: Add helper to retrieve command line options

Command line options passed to the compositors may change depending on what features were enabled at build time.

If mutter was built without X11 support, the command line option '--no-x11' may not be available.

Check if it is supported before using it, to avoid a potential failure if mutter does not support that option.

That applies to both mutter and gnome-kiosk backends.

Closes: #9 (closed)