If a peer was specified on the command line w/o a port, then do not
connect to it if we have inbound connections from that IP.
This eliminates unnecessary (and ultimately failing) connection
attempts in the common scenario of several peers having been told
about each other on their command lines.
We represent a command line peer address with a TCPAddr, which avoids
back and forth conversion to string and extra fields to carry the "did
the original include a port?" information.