Adding the same filters over and over again used to be okay, because

Adding the same filters over and over again used to be okay, because
we would lose the extra filters.  Now, if a filter is added, it is run.
Unfortunately, this can cause an infinite loop, or it can cause request
headers to appear twice.  This commit removes two instances in the core
where we were inserting filters for a second and third time.  The bug
was that error responses were causing infinite loops.

This also removes the reset_filters function, which did the exact
same thing as add_required_filters.  The two functions were both called
in error conditions, which was part of what caused this bug.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93693 13f79535-47bb-0310-9956-ffa450edef68