The existing logic to evaluate multi header "$sent_http_*" variables,
such as $sent_http_cache_control, as previously introduced in 1.23.0,
doesn't take into account that one or more elements can be cleared,
yet still present in a linked list, pointed to by the next field.
Such elements don't contribute to the resulting variable length, an
attempt to append a separator for them ends up in out of bounds write.

This is not possible with standard modules, though at least one third
party module is known to override multi header values this way, so it
makes sense to harden the logic.

The fix restores a generic boundary check.

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.

The ngx_http_process_multi_header_lines() function is removed, as it is
exactly equivalent to ngx_http_process_header_line().  Similarly,
ngx_http_variable_header() is used instead of ngx_http_variable_headers().

Multi headers are now using linked lists instead of arrays.  Notably,
the following fields were changed: r->headers_in.cookies (renamed
to r->headers_in.cookie), r->headers_in.x_forwarded_for,
r->headers_out.cache_control, r->headers_out.link, u->headers_in.cache_control
u->headers_in.cookies (renamed to u->headers_in.set_cookie).

The r->headers_in.cookies and u->headers_in.cookies fields were renamed
to r->headers_in.cookie and u->headers_in.set_cookie to match header names.

The ngx_http_parse_multi_header_lines() and ngx_http_parse_set_cookie_lines()
functions were changed accordingly.

With this change, multi headers are now essentially equivalent to normal
headers, and following changes will further make them equivalent.

Previously, $http_*, $sent_http_*, $sent_trailer_*, $upstream_http_*,
and $upstream_trailer_* variables returned only the first header (with
a few specially handled exceptions: $http_cookie, $http_x_forwarded_for,
$sent_http_cache_control, $sent_http_link).

With this change, all headers are returned, combined together.  For
example, $http_foo variable will be "a, b" if there are "Foo: a" and
"Foo: b" headers in the request.

Note that $upstream_http_set_cookie will also return all "Set-Cookie"
headers (ticket #1843), though this might not be what one want, since
the "Set-Cookie" header does not follow the list syntax (see RFC 7230,
section 3.2.2).

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.