Add ioctl fcntl and termios

Hi, i've been trying to write my own tui library, and i've had to use the c apis for a few things like getting the terminal size, etc... and i came by this package a while ago and saw it didn't abstract on ioctl fcntl and termios, it'd be very nice if it did!

Start by reviewing the existing Swift System abstractions and how this package represents low-level POSIX calls and types. Determine the intended scope for ioctl, fcntl, and termios, including terminal-size use cases; done should mean the requested C APIs are consistently abstracted for Swift users, with coverage for the supported behavior.

Written by the indexing model from the issue text.