Previously, when connecting to a backend, if the read event handler was
called before the write event handler, and the received response triggered
a next upstream condition, then ngx_http_upstream_reinit() was not called
to clean up the old upstream context.  This had multiple implications.

For all proxy modules, since the last upstream response was not cleaned up,
it was mixed with the next upstream response.  This could result in ignoring
the second response status code, duplicate response headers or reporting
old upstream header errors.

With ngx_http_grpc_module and ngx_http_proxy_v2_module, ctx->connection
was left dangling since the object it referenced was allocated from the
last upstream connection pool, which was deleted when freeing last upstream.
This lead to use-after-free when trying to reuse this object for the next
upstream.

When connecting to a backend, the connection write event is triggered
first in most cases.  However if a response arrives quickly enough, both
read and write events can be triggered together within the same event loop
iteration.  In this case the read event handler is called first and the
write event handler is called after it.

SSL initialization for backend connections happens only in the write event
handler since SSL handshake starts with sending Client Hello.  Previously,
if a backend sent a quick plain text response, it could be parsed by the
read event handler prior to starting SSL handshake on the connection.
The change adds protection against parsing such responses on SSL-enabled
connections.

For instance, the last_buf flag is used in the http proxy module when
creating HTTP/2 requests to indicate the output is closed.  The flag
is inherited in ngx_output_chain() to a destination buffer when reading
the buffered request body.  Then it is used in the proxy output filter
to mark the last HTTP/2 DATA frame with END_STREAM.

The problem happens when reusing the destination buffer, such as to
re-read the buffered request body on next upstream, because this buffer
may contain a dirty last_buf value, which breaks sending HTTP/2 request
body in multiple output filter calls.

The flush and last_in_chain flags are cleared for consistency.

If a buffered request body wasn't fully sent, such as on early upstream
response or limited by flow control, unsent buffers could remain in the
input or busy chains when switching to the next upstream server.  This
resulted either in the invalid request sent or a stalled connection.

The fix is to reset chains similar to ngx_http_upstream_reinit().

Configuration testing includes binding to configured listen addresses
when opening referenced files.

Now all GitHub .md files will reside in the docs directory.
While expicitly listing all files might be better for clarity,
this eliminates the need to touch "zip" target every such time.
This includes a recently added SUPPORT.md in 367113670.

A total response length with multiple ranges can be larger than the
source response size due to multipart boundary headers.  This change
extends max ranges limit imposed in c2c3e3105 (1.1.2) by accounting
boundary headers.  Notably, this covers suspicious requests with a
lot of small ranges that have an increased processing overhead and
are susceptible to range based amplification attacks.

The limit disables ranges as long as a total response length comes
close to the source size, additionally penalizing small size ranges
on a large source size where a processing overhead prevails, while
leaving a room for more ranges on a small source size, such that it
should not affect well-behaving applications.  The limit can be
altered with the "max_ranges" directive.

Closes #988 on GitHub.