As per perlxs, C preprocessor directives should be at the first
non-whitespace of a line to avoid interpreting them as comments.

#if and #endif are moved so that there are no blank lines before them
to retain them as part of the function body.

Previously, after sending a header we always sent a last buffer and
finalized a request with code 0, even in case of errors.  In some cases
this resulted in a loss of ability to detect the response wasn't complete
(e.g. if Content-Length was removed from a response by gzip filter).

This change tries to propogate to a client information that a response
isn't complete in such cases.  In particular, with this change we no longer
pretend a returned response is complete if we wasn't able to create
a temporary file.

If an error code suggests the error wasn't fatal, we flush buffered data
and disable keepalive, then finalize request normally.  This allows to to
propogate information about a problem to a client, while still sending all
the data we've got from an upstream.

No semantic changes expected, though some checks are done differently.
In particular, the r->cached flag is no longer explicitly checked.  Instead,
we relay on u->header_sent not being set if a response is sent from
a cache.

The NGX_HTTP_CLIENT_CLOSED_REQUEST code is allowed to happen after we
started sending a response (much like NGX_HTTP_REQUEST_TIME_OUT), so there
is no need to reset response code to 0 in this case.

Checks were added to both buffered and unbuffered code paths to detect
and complain if a response is incomplete.  Appropriate error codes are
now passed to ngx_http_upstream_finalize_request().

With this change in unbuffered mode we now use u->length set to -1 as an
indicator that EOF is allowed per protocol and used to indicate response
end (much like its with p->length in buffered mode).  Proxy module was
changed to set u->length to 1 (instead of previously used -1) in case of
chunked transfer encoding used to comply with the above.

That is, by default we assume that response end is signalled by
a connection close.  This seems to be better default, and in line
with u->pipe->length behaviour.

Memcached module was modified accordingly.

In case of upstream eof, only responses with u->pipe->length == -1
are now cached/stored.  This ensures that unfinished chunked responses
are not cached.

Note well - previously used checks for u->headers_in.content_length_n are
preserved.  This provides an additional level of protection if protol data
disagree with Content-Length header provided (e.g., a FastCGI response
is sent with wrong Content-Length, or an incomple SCGI or uwsgi response),
as well as protects from storing of responses to HEAD requests.  This should
be reconsidered if we'll consider caching of responses to HEAD requests.

While here, redundant parentheses removed.  No functional changes.