Prodded by Clang Static Analyzer.

This allows to build nginx on macOS with -Wdeprecated-declarations.

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.

Previously, fully preread unbuffered requests larger than client body
buffer size were saved to disk, despite the fact that "unbuffered" is
expected to imply no disk buffering.

The save body filter saves the request body to disk once the buffer is full.
Yet in HTTP/2 this might happen even if there is no need to save anything
to disk, notably when content length is known and the END_STREAM flag is
sent in a separate empty DATA frame.  Workaround is to provide additional
byte in the buffer, so saving the request body won't be triggered.

This fixes unexpected request body disk buffering in HTTP/2 observed after
the previous change when content length is known and the END_STREAM flag
is sent in a separate empty DATA frame.