Fix treatment of regex backreferences.

r904765 only made half of the necessary changes to remove the use
of '&' as an alias for '$0' and allow to escape any character with a
backslash.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1307067 13f79535-47bb-0310-9956-ffa450edef68