When allocating a display number, we should be aware that
IANA only allow TCP displays up to :63. This PR adds that restriction in
to sesman.ini as a default, to prevent us allocating unavailable TCP
ports.
By default TCP ports are not enabled for X servers, but users can easily
change this if they wish to access X displays directly over the network.
This restriction is in addition to the MaxSessions limit already present
in sesman.ini
matt335672
06580ec448