Updated `module/dns': non-exhaustive search fixed

The search for the matching host resolution left out one case where
the `hnh' is the `shn', but the associated `fqdn' is `shn.dn'; i.e.,
there is no `usdn' in the middle.
Updated `module/remote': ssh proxy functionality

The code handling the ssh proxy has been segregated into its own set
of functions, and the proxy string can now have an options colon,
followed by a port number.

Updated the example file with a commented `USER_SSH_PROXY' example.
Updated `module/tunnel': cleanup
Unit test `xplm' added: tackles nima/site#XXX