Make QueryMonitor() slightly less insane, making it return TRUE if the
specified monitor exists, rather than always returning TRUE (which we
are uselessly checking, and then also checking if the specified monitor
exists)
(Note that EnumDisplayMonitors() doesn't seem to have meaningful way to
return errors, see 5940580f)
Also: Spamming the long UseMsg() after "Invalid monitor number" isn't very
helpful.
Also: If we are exiting in ddxProcessArgument() due to an error in
options, use a non-zero exit status.
b078e03410