files: Move ctty fixup to open_fd()

Fixup ctty right after it's open. It'll need for
merging tty files in single fds list: set ctty
before next fle in list is processed. See next
patch for the details.

v4: New

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>