Casts are believed to be not needed, since memcmp() has "const void *"
arguments since introduction of the "void" type in C89.  And on pre-C89
platforms nginx is unlikely to compile without warnings anyway, as there
are no casts in memcpy() and memmove() calls.

These casts were added in 1648:89a47f19b9ec without any details on why they
were added, and Igor does not remember details either.  The most plausible
explanation is that they were copied from ngx_strcmp() and were not really
needed even at that time.

Prodded by Alejandro Colomar.

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().