Can't use show_help as the RML has already been enabled, but we haven't successfully connected back to the HNP. So use opal_output instead and hardwire the message.

Refs #4908
oob_tcp_connection: print "<unknown>" instead of "NULL"

"NULL" doesn't meany anything to the user, and is somewhat confusing
to see in an error message.  "<unknown>" at least indicates that
there's an error, and we know who the peer is.