more configure-time support for dynamically loadable MPMs:

. axe --with-mpm="shared" hack, replace with --enable-mpms-shared={all|list}
. replace singular MPM_NAME with access to the list of enabled MPMs
. replace singular MPM_SUBDIR with list MPM_SUBDIRS
. enable OS/2 MPM in same manner as others with configure support instead of
  hard-coding in configure.in

Current state: MPMs are built as static archives (but not linked to httpd) with
--enable-mpms-shared, so they still have to be built with apxs to load 
dynamically.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@832228 13f79535-47bb-0310-9956-ffa450edef68