Use IPv4 format for IPv4-mapped addresses

Use IPv4 format for IPv4-mapped addresses

For instance, use "127.0.0.1" for IPv4 loopback instead of
"::ffff:127.0.0.1".

Simplifies use of econfig key privip: plain dotted decimal now just
works regardless of IPv6 use, no need to add the IPv4-mapped form.

Also affects how addresses are logged and shown to players, and nation
selector ip.  Nicer that way.