[terminal] keep track of terminal raw/cooked state

[terminal] keep track of terminal raw/cooked state

Instead of setting the terminal to unbuffered (raw) mode on every
write, keep track of whether it's unbuffered or not at the points
we open and close the terminal.

Deactivate already takes care to set back into buffered mode;
otherwise we can end up resetting the terminal mode under X causing
Enter to send X SIGQUIT.