Don't hang if "Use DNS" is off. (Bug 1318.) Thanks to Rasmus

Sten who reported the bug and proposed an alternate fix.
* src/server/connections.c (login_request): Don't hang if the "Use
DNS" option is false.  (Bug 1318).  Log a warning message if
isc_resolve_remote() fails.
(src/server/testsuite/lyskomd.0/bug-1318.exp): New file.  Test the
"Use DNS: off" configuration option.