Fail pty opens that ask for parity or 7 bits

[tallakt]

Fixes #226. A pty can't do parity or 7 data bits, so opening one with them worked once and then failed with :einval. Fail the first open too, and document it.

[fhunleth]

Thanks! I hadn't realized it had been 2 days. This is on my todo list and I hope to make a pass soon.

[fhunleth]

Hi @tallakt - I took a more generic approach to the fix in #232, but copied over your unit test from here. I think this should be good. Let me know if you have any concerns and if not, I'll merge the other one and make a release. Thanks again for the issue and taking the time to make the PR.

[fhunleth]

I went ahead and merged #232 with your test from here. If I missed the mark, please let me know. Thanks again for reporting the issue and the PR.