If second REPL connect to device, first connection hangs

[mfikes]

Connect a REPL to a device and then connect a second to the same. The second will usurp the first, leaving the first with a hung REPL state.

Ideally we would do one of

• prevent the second connection
• allow the second connection but close the first without hanging
• allow both connections (probably not a good idea owing to complexity and potential for compiler state corruption)