See #1686; this allows i2pd to pass the i2p address to Ergo, which may be
useful for moderation under some circumstances.

This didn't work correctly for IPv6 or custom nets.
/UNDLINE IP can temporarily be used to reset the throttle.

Support PROXY protocol v2, including ahead of plaintext connections

See discussion on #68.

Since allocating them is rare.

As of 3dc5c8de78, the PROXY-before-TLS proxy line
and any proxy information carried in X-Forwarded-For
do not count as the "first line", so an additional
client-supplied PROXY line would have been accepted.

If you were banned and the ban was only detected when you proxied
(because you were proxying from a DLINE'd IP), you'd get an incorrect
quit message: `QUIT: Bad or unauthorized PROXY command`. This propagates
the correct ban message as the quit line.

1. Handle PROXY lines with IPv6 addresses starting with ::
(similar to WEBIRC in issue #211)

2. Strip v6 mapping from v4 addresses when handling proxied IPs.