[krbdev.mit.edu #1472] test suite should try a little harder to get root shell

[Ken Raeburn via RT]

Currently, for the dejagnu tests that require root access to run the
servers, we try "rlogin myhost -l root", and give up if it fails.  We
should also try "rlogin myhost -l root -x", and maybe "slogin myhost
-l root", if the basic rlogin gets connection-refused, as it probably
will in most cases nowadays.