mod_proxy: Add ap_connection_reusable() for checking if a connection
is reusable as of this point in processing.

mod_proxy_fcgi uses the new API to determine if FCGI_CONN_CLOSE
should be enabled, but that doesn't change existing behavior
since the connection is currently marked for closure elsewhere
in the module.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1526189 13f79535-47bb-0310-9956-ffa450edef68