add set_string_slot_lower

NOTE!  I notice that log_error is being deleted here... and I'm too tired
to figure out what in UpdateHide is causing that.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80483 13f79535-47bb-0310-9956-ffa450edef68