Changed the content-length filter to allow streaming delivery

Changed the content-length filter to allow streaming delivery
of content to clients

With this new code, the C-L filter will compute a content-length
if there's an EOS in the first brigade passed to it.  For normal
static file requests, the response header will still include a C-L.
If there's no EOS in the first brigade passed to the C-L filter,
the filter will give up on setting the C-L header, in favor of
passing the data on to the next filter in a timely manner.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96557 13f79535-47bb-0310-9956-ffa450edef68