Rename ap_casecmpstr[n]() to ap_cstr_casecmp[n](), update with APR doxygen

This time, with merge-ability back into 2.4.x


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1747478 13f79535-47bb-0310-9956-ffa450edef68