Add baudrate constants

[berkowski]

The only portable way to set baudrates in termios structs is through the B* constants defined for each platform. Given the use of ctest in libc it was suggested in nix-rust/nix#518 that these constants might be better defined libc itself, with nix providing access through a type-safe enum.

Seems straight forward (prior art: https://github.com/dcuddeback/termios-rs) and fits the scope.