su: (pty) improve SIGSTOP/SIGCONT semantic

su: (pty) improve SIGSTOP/SIGCONT semantic

We want to use waitpid() only when child is terminated or stopped to
pick up child status, otherwise PTY proxy has to be active. This is
difference between "su" and "su --pty". For "su" we keep parent all
time in waitpid().

It would be possible to use separate code based on signalfd_siginfo,
but it seems better to keep all this stuff on one place -- it  means
wait_for_child().

Signed-off-by: Karel Zak <kzak@redhat.com>