Simplify "setproxy" target.

The "proxy.use" flag can now be "on", "yes" or "true", so document "true" as the recommended value.
The property expansion feature for if/unless attributes is since Ant 1.8.0.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1606128 13f79535-47bb-0310-9956-ffa450edef68