Overhauled mod_rewrite's general substitution function

(expand_backref_inbuffer): 1. The `$0' backreference is now officially allowed
and documented and references the while pattern space; 2. the ampersamp (&)
backreference (which is equal to $0) is no longer expanded, because it was
never documented and only leads to confusion with QUERY_STRINGS; 3.
backslashes (\) are honored correctly, that is `\$N' now really forces the
dollar to be an ordinary character and $N is not expanded.

Submitted by: Ralf S. Engelschall
PR: 4766 4161


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@84049 13f79535-47bb-0310-9956-ffa450edef68