allow users to workaround the over-agressive backreference
escaping by selecting the characters to escape. 



add BNP flag to give control to the user on whether a space ' ' in
an escaped backrefernece is decoded to a + (default) or %20. Useful
if your backreference isn't going into the query string.


Submitted by: covener
Reviewed by: jailletc36, covener, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1796850 13f79535-47bb-0310-9956-ffa450edef68