tweak MPM configure to avoid having to grep for MPM names in lists

of MPMs of some type (threaded or share-able or enabled), as suggested
by jorton

rename some MPM-related variables


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@832907 13f79535-47bb-0310-9956-ffa450edef68