login: improve work with signals

login: improve work with signals

The login cannot ignore signals, because:

 * SIGHUP is only way how inform session leader that controlling
   tty goes away. The leader has to inform others processes in same
   process group about the signal.

 * SIGHUP/SIGTERM cannot kill wait(2)-ing login, we have to wait as long
   as any child process exists. The PAM session has to be closed correctly.

 * The child process (before setsid()) has to call exit() if a controlling
   tty goes away.

This patch is inspired by patch from Red Hat that is very well tested for last
4 years in all Red Hat distros.

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