Fix some IPv6 address compatibility issues.

src/proto-p10.c (irc_p10_pton): Mark valid IPv4 addresses as v4-mapped.
  (irc_p10_ntop): Emit invalid IPs as AAAAAA (but it could be _ for +6
    flagged hubs).
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-11