Provide our own impl of str[n]casecmp()

This simply provides it. Next step is to change all uses of
str[n]casecmp to ap_str[n]casecmp and *then* remove those silly
logic paths where we check the 1st char of a string before
we do the strcasecmp (since this is no longer expensive).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1715401 13f79535-47bb-0310-9956-ffa450edef68