The variables have prefix $proxy_protocol_tlv_ and are accessible by name
and by type.  Examples are: $proxy_protocol_tlv_0x01, $proxy_protocol_tlv_alpn.

New variables $proxy_protocol_server_addr and $proxy_protocol_server_port are
added both to HTTP and Stream.

Now a new structure ngx_proxy_protocol_t holds these fields.  This allows
to add more PROXY protocol fields in the future without modifying the
connection structure.

No functional changes.

Previously, unix sockets were treated as AF_INET ones, and this may
result in buffer overread on Linux, where unbound unix sockets have
2-byte addresses.

Note that it is not correct to use just sun_path as a binary representation
for unix sockets.  This will result in an empty string for unbound unix
sockets, and thus behaviour of limit_req and limit_conn will change when
switching from $remote_addr to $binary_remote_addr.  As such, normal text
representation is used.

Reported by Stephan Dollberg.

No functional changes.

Unlimited recursion might cause stack exhaustion in some misconfigurations.