Fixes for USB serial on Linux

[SafariMonkey]

Description

Fixes to make serial run on Linux:

• Allow "/dev/tty" as an alternate prefix to COM for serial source detection
• Don't try to set buffer size outside Windows because the function just doesn't exist.

A couple choices I made that I would like a confirmation on, but seemed reasonable:

• COM is now tested as a prefix, not an "in".
• I changed from checking the UI input for len < 5 to checking for COM prefix. If leaving off the COM is allowed, the length check could be left in, but I couldn't find anywhere that added COM later.
• I test /dev/tty as the non-Windows prefix. This is apparently not entirely universal, e.g. see here, but the exceptions listed are all BSDs. /dev/ would catch all those, but also /dev/video* and so webcam hardware would get caught in that too. (I checked, it can capture e.g. the Index cameras, as long as the http prefixing is bypassed for that.) One option if you wanna allow for the cases where auto-detection fails could be to add a toggle or dropdown next to the input box, maybe with a default "auto-detect" that detects most cases. However, I figured I'd improve over the current situation and leave anything further to your discretion.

Checklist

• I accept the CLA.