Finish the MPM decision logic. This moves the threads checking stuff into

the mpm directory where it belongs.  It also adds logic so that if threads
aren't detected, we automatically choose to use the prefork mpm.  Lastly,
I also cleaned up a variable name to make it more consistent with the other
variables used in our configure script.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84365 13f79535-47bb-0310-9956-ffa450edef68