Add termios2 support

[philj56]

This adds support for termios2 on Linux, covered by an autoconf check.

There's a small workaround to allow functions from <termios.h> to still be used (i.e. tcflush()), but it could be removed and tcflush() replaced with an ioctl() call if preferred.

An additional check is made in debug mode to see if the desired baud rate has correctly been set; I don't think there's another way to know this, but I'm not 100% sure.

This shouldn't change anything for other platforms - I've tested this on a Mac, but not BSD as I don't have one available.

[cla-bot]

We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, please fill https://forms.gle/5635zjphDo5JEJQSA to get added. Your document will be manually checked by the maintainer. Be patient...

[quozl]

Tested 52abfc1 cherry-picked onto bb13443 on Linux 6.8.0 with an FT232R to a control system with five nodes configured for 256000 baud.

[stephane]

Thank you @quolz for the test and @philj56 for the patch but I need you CLA to merge your patch.

[quozl]

Best of luck with that. I'll continue to use a fork.

In your CLA, there's a typo "Lilcense". Suggest you update CONTRIBUTING.md and README.md to refer to CLA. This suggestion is made without agreement to the CLA.

Cost of legal review is too high given the expected return on investment for me right now.

Quick fix would be for @philj56 to make a copyright assignment rather than CLA.

[philj56]

@stephane I think I already did agree to the CLA before, but I've filled the form in again just now.

[stephane]

Thank you. Tested and merged.