The directive allows to pass upstream response trailers to client.

The new condition produces smaller diff to the default branch and is similar to
HTTP/2 case.

If a filter wants to buffer the request body during reading (for
example, to check an external scanner), it can now do so.  To make
it possible, the code now checks rb->last_saved (introduced in the
previous change) along with rb->rest == 0.

Since in HTTP/2 this requires flow control to avoid overflowing the
request body buffer, so filters which need buffering have to set
the rb->filter_need_buffering flag on the first filter call.  (Note
that each filter is expected to call the next filter, so all filters
will be able set the flag if needed.)

It indicates that the last buffer was received by the save filter,
and can be used to check this at higher levels.  To be used in the
following changes.

If due to an error ngx_http_request_body_save_filter() is called
more than once with rb->rest == 0, this used to result in a segmentation
fault.  Added an alert to catch such errors, just in case.

Do this only when the entire request body is empty and
r->request_body_in_file_only is set.

The issue manifested itself with missing warning "a client request body is
buffered to a temporary file" when the entire rb->buf is full and all buffers
are delayed by a filter.

The change reduces diff to the default branch for
src/http/ngx_http_request_body.c.

Also, client Content-Length, if present, is now checked against the real body
size sent by client.