Output chain: clear the last_buf flag unless inherited.

Output chain: clear the last_buf flag unless inherited.

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.