Allow (a hokey) opt-in to connection reuse for mod_proxy_fcgi + TCP.

Connection reuse has been disabled since r1032345 at the end of
2011. 

Attempt to reverse the polarity of the connection reuse doc which
has been wrong for a long time.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1647005 13f79535-47bb-0310-9956-ffa450edef68