svc/conntrack.in: Make an `InetAddress' class to do address wrangling.

The name is a little misleading: it can also represent a network, but
separating the two turns out to be a little tedious, so I don't bother.

This means that the configuration now actually contains (PEER,
TEST-ADDRESS, LOCAL-NET) triples, rather than keeping the address and
mask portions of the LOCAL-NET separate.

This is rather an invasive change.  Sorry.