Initial version of the mpmt MPM. This MPM can emulate dexter, mpmt_pthread,

and prefork.  This basically just combines a lot of common code.  This
builds and serves pages in all three modes, but I don't think killing
the server works in any of them.  The configuration system hasn't changed
at all, so using --with-mpm=(prefork|dexter|mpmt_pthread) all work.

My goal is to remove the three separate MPMs in a few days, and then work
on merging one of the BeOS and the OS/2 MPMs into this as well.  :-)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85788 13f79535-47bb-0310-9956-ffa450edef68