Replace the "tty" driver with a "pty" driver.

This new driver uses the "generic" TTY layer, unlike the old driver
which had its own implementation (which the generic module was derived
from, originally.)

The remaining bits of support for controlling TTYs is added to the kernel &
generic layer at the same time, which should allow for serial interfaces
to be controlling terminals now, as well.

Tested with bash, nano, vim; all seems to still be working as expected.