su: print proper core dump message when killed

An example is, in one terminal "sudo su -; echo $$", and in
another terminal, "kill -9  $PID" (the pid of the su -). It
should not print "(core dumped)", unless the kill signal
specified so, e.g. kill -7 or kill -11.

Signed-off-by: pcpa <paulo.cesar.pereira.de.andrade@gmail.com>