Handle -displayfd and an explicit display number sensibly, e.g. use the
explicitly specified display number, and write it to the displayfd
v2: displayfd might be 0, so use -1 as invalid value
v3: Rebase for addition of NoListenAll flag
Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>