Asyn: add a "break on every write" option.

[Tom-Willemsen]

A serial break is not a series of characters but a fundamental "thing" you can do to a serial line.

This is required for ticket #2397 (fermi chopper).

Plan is to implement an option in the asyn driver that sends a serial break on the end of every message.

[FreddieAkeroyd]

• Added a "break" option to asynSetOption - specify break length in milliseconds
• A break will be asserted after every write

To test:

• if kit requiring a serial break is not available, try it on existing equipment to see what happens
• enable ASYN_TRACE - you should see the words "with BREAK" displayed when number of bytes written is printed
• you might be able to see a break with a serial port sniffer

[Tom-Willemsen]

Tested by:

• Connected to eurotherm without asyn break option - behaviour was unchanged
• Connected to eurotherm with new asyn break option - things started timing out indicating that something was happening differently.
• Enabled asyn trace and the new message was correctly printed ("with BREAK")
• We don't have access to a device that requires serial breaks so I can't test that it works 100% correctly but I've reviewed the code and it looks sensible.