Merge r1664565 from trunk:

*) mod_rewrite: Add support for starting External Rewriting Programs
   as non-root user on UNIX systems by specifying username and group name
   as third argument of RewriteMap directive.

Submitted by: jkaluza
Reviewed by: jorton, wrowe, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1811748 13f79535-47bb-0310-9956-ffa450edef68