PTY: merge pty_install implementations

There are currently two instances of code which handles PTY install.
One for the legacy BSD PTY's, one for unix98's PTY's. Both of them are
very similar and differ only in termios allocation and handling.

Since we will need to allocate a tty_port at that place, this would
require editing two places with the same pattern. Instead, let us move
the implementation to one common place and call it from both places.

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