su: don't use kill(0, ...) when propagate signal

The current code uses kill(0, caught_signal) after regular
signal cleanup and before exit (all just to make shells happy).

Unfortunately, kill(0, ...) is a bad idea. It seems better to use
kill(getpid(), ...) otherwise we kill our parent process too.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1063887
Signed-off-by: Karel Zak <kzak@redhat.com>