Feature: port mode change for NX2 server

[lkarsten]

The NX2 server starts up with its serial interface in NMEA0183 mode at 4800 baud.

By sending a special NMEA0183 message ( it can be configured to run at 19200 baud.

Another NMEA0183 message will reconfigure the port from NMEA0183 to FDX.

Status:

• Support for mode change into FDX was added in 73df855 with "--send-psilfdx", but is so far untested on real hardware. Depending on if the NX2 server is reading the data, multiple restarts of fdxread with this option may lead to blocking writes, I don't know. Serial ports are finicky things.

• Changing the data rate isn't added yet. I'm not sure if we need it. I can think of a whole lot of edge cases when this speed change is happening, especially if we are changing mode to FDX almost at the same time. With some luck this complexity can be avoided. (Q: does mode change to FDX also change the port speed? Will --send-psilfdx need to close/reopen the port to get going again?)