mod_proxy_fcgi: Add the support for mod_proxy's

mod_proxy_fcgi: Add the support for mod_proxy's
                flushpackets and flushwait params

This change was requested on the development mailing
list in order to fill another gap between mod_fcgi
and mod_proxy_fcgi, namely the -flush funtionality.

The more evolved core trunk code would not need this
feature becuse of the non-blocking writes, but it
is be needed in 2.4.x.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1802040 13f79535-47bb-0310-9956-ffa450edef68