Add support for sending serial break via option interface.

[lrossa]

The asyn option interface is used to set/clear a break state on serial lines.

Thanks to idea how to do it under Microsoft Windows are taken from here:
Freddie Akeroyd freddie.akeroyd@stfc.ac.uk
https://github.com/ISISComputingGroup/EPICS-asyn
commit 1228f8ea40fcc1060fd996b4a8669c3572d1d4ba (Tue Nov 14 12:57:31 2017 +0000)
commit a8496cf53a4b138469a639cb7bbc59caea8c3ec6 (Tue Nov 14 13:08:37 2017 +0000)

[AppVeyorBot]

✅ Build asyn 1.0.227 completed (commit 89aa78f4c3 by @lrossa)

[AppVeyorBot]

✅ Build asyn 1.0.229 completed (commit 38450c7367 by @lrossa)

[FreddieAkeroyd]

Hi @irossa in ISISComputingGroup/EPICS-asyn#5 a serial break was applied for a fixed duration automatically at the end of a writeIt as we had a device that used this as a sort of command terminator. If I understand correctly, in your case you only need to apply/clear a serial break via a setOption command? I'd like to submit a PR to add this automatic break functionality too, I wasn't sure of the best naming scheme though. I could use autobreak and "yes/no/number" like your break command, or add a break_len option and break then takes "on/off/auto". Thank you for adding RFC2217 support in asynInterposeCom.c - we don't use that protocol at the moment, but it is something we have been looking at.

[FreddieAkeroyd]

Does something along the lines of #188 look reasonable?