Upstream: drop extra data sent by upstream.

Upstream: drop extra data sent by upstream.

Previous behaviour was to pass everything to the client, but this
seems to be suboptimal and causes issues (ticket #1695).  Fix is to
drop extra data instead, as it naturally happens in most clients.

This change covers generic buffered and unbuffered filters as used
in the scgi and uwsgi modules.  Appropriate input filter init
handlers are provided by the scgi and uwsgi modules to set corresponding
lengths.

Note that for responses to HEAD requests there is an exception:
we do allow any response length.  This is because responses to HEAD
requests might be actual full responses, and it is up to nginx
to remove the response body.  If caching is enabled, only full
responses matching the Content-Length header will be cached
(see b779728b180c).