The check is not expected to fail unless there is a bug in the calling
code.  But given the check is here, it should log an alert if it fails
instead of silently closing the connection.

Maximum size for reading the PROXY protocol header is increased to 4096 to
accommodate a bigger number of TLVs, which are supported since cca4c8a715de.

Maximum size for writing the PROXY protocol header is not changed since only
version 1 is currently supported.

The cast is added to make ngx_proxy_protocol_parse_uint16() similar to
ngx_proxy_protocol_parse_uint32().

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.

Previously, all received user input was logged.  If a multi-line text was
received from client and logged, it could reduce log readability and also make
it harder to parse nginx log by scripts.  The change brings to PROXY protocol
the same behavior that exists for HTTP request line in
ngx_http_log_error_handler().

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.

Both Solaris and Windows define "s_addr" as a macro.

- use normal prefixes for types and macros
- removed some macros and types
- revised debug messages
- removed useless check of ngx_sock_ntop() returning 0
- removed special processing of AF_UNSPEC