compositor-headless: Support starting with zero outputs

compositor-headless: Support starting with zero outputs

This patch adds a new command line option which can be
used to tell headless backend not to create any
virtual outputs.

This will be used for output hotplug emulation, where
weston will start with no outputs available, and the
virtual output will be created at runtime.

v2:

- Use bool instead of int for the indicator flag
- Move final newspace to a separate line in command
  line options

Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>