This is just too easy to not do it: Add an 'expr' authz provider that allows

This is just too easy to not do it: Add an 'expr' authz provider that allows
arbitrary expressions in Require lines.

The main issue I wanted to fix was that the env provider only allows to
check for the existance of an envvar but not the contents.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1002363 13f79535-47bb-0310-9956-ffa450edef68