Fix the mpmt_pthread MPM to use an APR pipe for the pipe of death. This

allows us to use APR_SOCKETS_AS_FILES to poll on an APR socket.  Finally,
this makes the MPM more portable, because it is now always possible to set
the pipe to be non-blocking.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85676 13f79535-47bb-0310-9956-ffa450edef68