The h->next pointer properly provided as NULL in all cases where known
output headers are added.

Note that there are 3rd party modules which might not do this, and it
might be risky to rely on this for arbitrary headers.

If a memcached response was followed by a correct trailer, and then
the NUL character followed by some extra data - this was accepted by
the trailer checking code.  This in turn resulted in ctx->rest underflow
and caused negative size buffer on the next reading from the upstream,
followed by the "negative size buf in writer" alert.

Fix is to always check for too long responses, so a correct trailer cannot
be followed by extra data.

The directives enable the use of the SO_KEEPALIVE option on
upstream connections.  By default, the value is left unchanged.

The fields "uri", "location", and "url" from ngx_http_upstream_conf_t
moved to ngx_http_proxy_loc_conf_t and ngx_http_proxy_vars_t, reflect
this change in create_loc_conf comments.

This parameter lets binding the proxy connection to a non-local address.
Upstream will see the connection as coming from that address.
When used with $remote_addr, upstream will accept the connection from real
client address.

Example:

    proxy_bind $remote_addr transparent;

The new directives {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_tries
and {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_timeout limit
the number of upstreams tried and the maximum time spent for these tries
when searching for a valid upstream.