Add ap_regexec_len() function that works with non-null-terminated

strings.

PR: 51231
Submitted by: Yehezkel Horowitz <horowity checkpoint com>, Stefan Fritsch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1125802 13f79535-47bb-0310-9956-ffa450edef68