embed ipv4 source in ipv6

[candlerb]

Proof-of-concept for #371: for review rather than actual merge.

Usage:

listen 0.0.0.0 443 {
    proto tls
    embed fd46:1::
}

If the inbound connection is IPv4, and the outbound connection is IPv6, then the incoming source IPv4 address is embedded in the low 32 bits of the outgoing IPv6 source.

[candlerb]

I am now wondering if this would be more logical if configured as a per-backend option, like proxy_protocol. e.g.

embed_prefix fd46:1::

table foo {
    bar.example.net  *:4443  proxy_protocol
    baz.example.net  *       embed
}

(although the way I do it now is consistent with the "source" option on the listener)