Serial Multiplexer cleanup after review by Dave Bryan

- Added functionality to return the lines with attached serial ports by calling tmxr_poll_conn one time after a serial port is attached.
- Added the ability to close a serial port from the sim> prompt if a device implements a SET dev DISCONNECT=line command.  A serial port is closed if the -C switch is specified on the DISCONNECT command line.
- Cleaned up the multiplexer status display based on Dave's recommendations.
2 files changed