PTY: add tty_port

This has *no* function in the PTY driver yet. However as the tty
buffers will move to the tty_port structure, we will need tty_port for
all TTYs in the system, PTY inclusive.

For PTYs this is ensured by allocating 2 tty_port's in pty_install,
i.e. where the tty->link is allocated. Both tty_port's are properly
assigned to each end of the tty.

Freeing is done at the same place where tty is freed, i.e. in
tty->ops->cleanup.

This means BTW that tty_port does not outlive TTY in PTY. This might
be a subject to change in the future if we see some problems.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>