SCP/SOCKET: Apply default host more reasonably when parsing host:port strings
TMXR: Tolerate situations where independent devices are bound to different lines of a multi-line MUX.
H316: Rewrote h316_udp to use TMXR lines for UDP transport of data.  Changed h316_mi to leverage built-in loopback mode in TMXR.

This gives UDP transport on all simh host platforms.
TMXR: Support loopback when using packet APIs even if a connection is not active