Make ap_pregsub(), used by AliasMatch and friends, use the same syntax

for regex backreferences as mod_rewrite and mod_include: Remove the use
of '&' as an alias for '$0' and allow to escape any character with a
backslash.

Document the use of $0.

PR: 48351


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@904765 13f79535-47bb-0310-9956-ffa450edef68