Rework network port handling.

Unless acting as a server, let the system select port numbers.
Client binaries will re-open their sockets when entering and leaving server mode.
Client port numbers are not persistent across server mode.

Servers will not try to open ports < 1024.

The “opening socket” messages are reworked a little.

PORT_ANY is now defined as 0 to match standard networking usage.

Currently in-use port nos. are in net_currentPort and net_currentPort6.