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.)

Bring OS/2 MPM back from the dead. Reverses r758899.

remove OS/2 MPM

New multi-process multi-threaded MPM for OS/2. Not fully polished but works better than spmt_os2 already (graceful restarts actually work). A summary of the process/thread structure is provided in the comments at the start of mpmt_os2.c