Change the configure-based MPM build mechanism to support building an MPM as a shared shared or dynamic module, primarily using the APACHE_MPM_MODULE() function. --enable-mpms-shared now builds/installs the MPMs as dynamic modules. (But no LoadModule directives are added.)

The Event MPM is no longer experimental; move it out of that ghetto. Yank the few bits that implement the concept of an experimental MPM.