Follow up to r1840265: really privatize ap_filter_{recycle,adopt_brigade}().

Follow up to r1840265: really privatize ap_filter_{recycle,adopt_brigade}().

Move ap_filter_adopt_brigade()'s declaration to "server/core.h" (private).

For ap_filter_recycle(), make it static/internal to util_filter (renamed to
recycle_dead_filters() which better fits what it does). It's now also called
unconditionally from ap_filter_input_pending() which itself is always called
after the request processing and from MPM event (as input_pending hook).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1840611 13f79535-47bb-0310-9956-ffa450edef68