Introduce a new set of APIs to allow MPMs to be proper modules instead

of integral parts which share global variables, functions, and macros
with the rest of httpd.

Converted now:
  prefork, worker, event, simple, WinNT*

*WinNT hasn't been built or tested, and relies on a hack to include the
WinNT mpm.h to disable Unixy MPM support routines in mpm_common.c


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@757853 13f79535-47bb-0310-9956-ffa450edef68