(tty::is_master_closed): Drop method.
* fhandler_tty.cc (fhandler_pty_slave::open): Remove code prematurely
bailing out if master control thread is not running.
(fhandler_pty_slave::read): Don't generate SIGHUP if master control
thread is not running.
(fhandler_pty_master::close): Rearrange code to avoid stopping master
control thread twice in multi-threaded scenarios.