tty: Refactor session leader SIGHUP from __tty_hangup()

Reduce complexity of __tty_hangup(); separate SIGHUP signalling
into tty_signal_session_leader().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>