Update termios bindings

[hovsater]

This includes several improvements to the existing termios bindings.

• The dummy interface was removed as requested by @lerno.
• The existing bindings were moved from lib/std/libc/os/posix.c3 into lib/std/libc/os/linux.c3 as they were Linux specific.
• The type of the c_cc member in the Termios struct was adjusted for the Linux bindings as agreed upon with @ManuLinares.
• New Darwin specific bindings were added.

[ManuLinares]

I don't know about the alias Cc = char, maybe just make it a char directly instead

[hovsater]

I don't know about the alias Cc = char, maybe just make it a char directly instead

I don't mind changing it. The reason for the alias is because that's what both Odin and Zig does (since termios exposes a cc_t type). On Linux, Cc is also used for multiple fields (c_cc[NCCS] and c_line).