kboot: implement stripped down termios

Implement a stripped down termios, obtained from various files in musl
and HOST_ or host_ prepended to most things and a few unavoidable style
tweaks. Only implements the bits of termios we need for the boot loader:
put the terminal into raw mode, restore terminal settings and speed
stuff.

Sponsored by: Netflix

(cherry picked from commit 963037786f76a414e91e45d4fbef5e35fc1abc84)