Added support for network based CUL devices

This works with a CUN/CUNO device,
or a CUL device that has its serial connection
forwarded with eg. socat to a TCP port.

Usage example for FHT:
fht:device=network:192.168.178.23:2323

You can leave out the port, in that case the
default 2323 used by the CUN/CUNO is used.