Move ap_set_last_modified to the core. This is a potentially

Move ap_set_last_modified to the core.  This is a potentially
controversial change, because this is kind of HTTP specific.  However
many protocols should be able to take advantage of this kind of
information.  I expect that headers will need one more layer of
indirection for multi-protocol work, but this is a small step in
the right direction.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88580 13f79535-47bb-0310-9956-ffa450edef68