Add flags missing from termios

[gergelyk]

Adding flags from c_cc characters section. Refer to https://sites.uclouvain.be/SystInfo/usr/include/bits/termios.h.html

[ysbaddaden]

There are a few issues to solve beforehand:

• these are non standard (as per https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/termios.h.html);
• there are only bindings for Linux while the other BSD targets also have most of them;
• last but not least, we won't use them in stdlib, and we don't intend to support everything from the C library in the compiler.

[gergelyk]

I agree. The only flag that I was missing in this project in fact was VTIME. It seems to be part of non-canonical mode, like VSTART, VSTOP, etc. What do you think? Should this PR include only VTIME?

[straight-shoota]

There's an ongoing discussion about the status of libc bindings in stdlib (#13504). The general idea is that we don't plan to support any bindings that are not used in stdlib itself. Unused bindings might be removed in the future (#11955). So I don't think it's a good idea to add more unused bindings at this point.
This discussion is not finialized and might come to a different conclusion after all (though I doubt it).

If you need specific bindings for your project, it's probably best to define them in your own code.

[ysbaddaden]

(I really should just restart my posix shard as a libc shard, with libvirt VMs to autogenerate the bindings for all supported targets)