Bring forward from 1.3:

I broke mod_rewrite by modifying strings in place when expanding them,
because variable lookups can cause subrequests which cause mod_rewrite
to do its stuff again including an expansion on the same string, which
is then syntactically invalid. So copy the lookup keys somewhere else
before using them in such a way that may cause recursion.

In addition to this, my parser could also be confused by complicated
nested rewrite map expansions like ${map1:${map2:key|dflt}|dflt} so
fix that too by keeping track of {} when looking for |.

PR:             7087


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87723 13f79535-47bb-0310-9956-ffa450edef68