Add ap_cstr_casecmp[n]() - placeholder of apr_cstr_casecmp[n] functions

for case-insensitive C/POSIX-locale token comparison.

Submitted by: wrowe
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1748334 13f79535-47bb-0310-9956-ffa450edef68