Fix flags for FreeBSD/arm64

[timstpierre-tc]

Couldn't get this package to work with FreeBSD on arm64, and eventually figured out why.

• Setting the Control flags with the baud rate seems to break things, specifically the word size. Removing that solved the issue.
• The post-processing output flag is needed. Turning that flag off broke newline handling and control codes. In order to turn that on only for FreeBSD, a constant was added with set and clear values that can then be added in each os-specific file. I set the correct values for FreeBSD and then set zero values in the other UNIX os files. Hoping this should keep it working everywhere else (since I also tested on linux/arm64 and darwin/arm64 and it worked fine there).

-Confirmed setting baud rate and parity work correctly with the changes on FreeBSD/arm64.