Generic subrequests in memory.

Previously, only the upstream response body could be accessed with the
NGX_HTTP_SUBREQUEST_IN_MEMORY feature.  Now any response body from a subrequest
can be saved in a memory buffer.  It is available as a single buffer in r->out
and the buffer size is configured by the subrequest_output_buffer_size
directive.

Upstream, proxy and fastcgi code used to handle the old-style feature is
removed.