DUNE/Hardware/SerialPort: add read-only option

[krisgry]

Small PR to make it possible to use the SerialPort interface in read-only mode. Work by Artur Zolich.

Also have some code that makes use of this in SerialOverTCP, but that code is a bit "brute force". Just mentioning it, in case someone is interested.

The use-case was: two UHF radios on the ground, and wanted to be absolutely sure that only one of them was sending data.