Instead of having wayvnc/neatvnc create a socket and listen() on it,
allow listening on an already bound socket file descriptor with the
--external-listener-fd=FD option. This may be used to support any kind of
connection-based socket that isn't explicitly supported by wayvnc in an option,
such as ones using the AF_VSOCK address family, or support cases where wayvnc
is being activated by systemd (via a .socket unit) or a super-server like
inetd.