upnp: More accurate getOurIP.
upnp: More accurate getOurIP.

This makes getOurIP return a local IP that is on the same subnet as
the serviceIP used for uPnP.  Verified getOurIP doesn't return a local
IPv6 IP when the serviceIP is IPv4.  Didn't test with IPv6 serviceIP.

Co-authored-by: lol_and_hold <63636f1ca9162121395f8f0761f105af69539a69@protonmail.com>
decred/dcrd,decred/dcrd