pt_chown: Clear any signal mask inherited from the parent process.

If grantpt() is called from a thread that is masking signals (for
instance, from a program using signalfd or using a dedicated
signal-handling thread), then that mask will get inherited to pt_chown.
This means that signals like SIGINT will not interrup pt_chown, so if it
hangs (e.g., because getgrnam("tty") hangs on a remote name service),
Ctrl-C will terminate the parent process but leave pt_chown around. Since
it's setuid, it's hard to kill any other way.

It is safe for pt_chown to unmask all signals, because grantpt() can be
(and usually is) called from an unprivileged process with all signals
unmasked.