directory_walk() is an expensive function, keep a little more state to

avoid needless string counting.  Add two new functions make_dirstr_parent
and make_dirstr_prefix which replace all existing uses of make_dirstr.
The new functions are a little less general than make_dirstr, but
work more efficiently (less memory, less string counting).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@78479 13f79535-47bb-0310-9956-ffa450edef68