BUG/MINOR: connection: make proxy protocol v1 support the UNKNOWN protocol

If haproxy receives a connection over a unix socket and forwards it to
another haproxy instance using proxy protocol v1, it sends an UNKNOWN
protocol, which is rejected by the other side. Make the receiver accept
the UNKNOWN protocol as per the spec, and only use the local connection's
address for this.
DOC: proxy protocol example parser was still wrong

Now that version and cmd are in the same byte, it is not possible
anymore to compare the version as a 13th byte.
DOC: minor updates to the proxy protocol doc

Update the release data, revision history and the link to the Forwarded
HTTP extension.
CLEANUP: connection: merge proxy proto v2 header and address block

This is in order to simplify the PPv2 header parsing code to look more
like the one provided as an example in the spec. No code change was
performed beyond just merging the proxy_addr union into the proxy_hdr_v2
struct.
MEDIUM: connection: add support for proxy protocol v2 in accept-proxy

The "accept-proxy" statement of bind lines was still limited to version
1 of the protocol, while send-proxy-v2 is now available on the server
lines. This patch adds support for parsing v2 of the protocol on incoming
connections. The v2 header is automatically recognized so there is no
need for a new option.