xfree86/modes: Add "NoOutputInitialSize" option

Normally, the X server infers the initial screen size based on any
connected outputs.  However, if no outputs are connected, the X server
picks a default screen size of 1024 x 768.  This option overrides the
default screen size to use when no outputs are connected.  In contrast
to the "Virtual" Display SubSection entry, which applies unconditionally,
"NoOutputInitialSize" is only used if no outputs are detected when the
X server starts.

Parse this option in the new exported helper function
xf86AssignNoOutputInitialSize(), so that other XFree86 loadable drivers
can use it, even if they don't use xf86InitialConfiguration().

Signed-off-by: Andy Ritger <aritger@nvidia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
3 jobs for no-output-initial-size in 6 minutes and 55 seconds (queued for 2 minutes and 24 seconds)