If proxy_pass (and friends) with variables evaluates an upstream
specified with literal address, nginx always created a per-request
upstream.

Now, if there's a matching upstream specified in the configuration
(either implicit or explicit), it will be used instead.

This fixes inconsistency in what is stored in the "host" field.
Normally it would contain the "host" part of the parsed URL
(e.g., proxy_pass with variables), but for the case of an
implicit upstream specified with literal address it contained
the text representation of the socket address (that is, host
including port for IP).

Now the "host" field always contains the "host" part of the URL,
while the text representation of the socket address is stored
in the newly added "name" field.

The ngx_http_upstream_create_round_robin_peer() function was
modified accordingly in a way to be compatible with the code
that does not know about the new "name" field.

The "stream" code was similarly modified except for not adding
compatibility in ngx_stream_upstream_create_round_robin_peer().

This change is also a prerequisite for the next change.

The first condition added in d3454e719bbb should have just replaced
the second one.

BoringSSL changed SSL_set_tlsext_host_name() to be a real function
with a (const char *) argument, so it now triggers a warning due to
conversion from (u_char *).  Added an explicit cast to silence the
warning.

Prodded by Piotr Sikora, Alessandro Ghedini.

External structures are now identical regardless of stream SSL module
compiled in or not.

Made sure to set the variable length only after successful SNI parsing.

Removed NGX_HAVE_REUSEPORT from the signature accordingly.

The IPV6_V6ONLY macro is now checked only while parsing appropriate flag
and when using the macro.

The ipv6only field in listen structures is always initialized to 1,
even if not supported on a given platform.  This is expected to prevent
a module compiled without IPV6_V6ONLY from accidentally creating dual
sockets if loaded into main binary with proper IPV6_V6ONLY support.

When enabled, some structures are padded to be size compatible
with their NGINX Plus versions.

It is to be used as a bitmask with various bits set/reset when appropriate.
Any bit set means that the peer should not be used, that is, exactly what
current checks do, no additional changes required.