Fix usage of MOD_* variables for MPM modules

r1385216 broke building of mpm event with serf support. To fix,

* introduce APACHE_MPMPATH_INIT/FINISH similar to
  APACHE_MODPATH_INIT/FINISH to create modules.mk and add
  MOD_* vars to it.
* change APACHE_MPM_MODULE to only append to modules.mk, not
  overwrite
* add APACHE_MPMPATH_INIT/FINISH to all MPMs


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1395252 13f79535-47bb-0310-9956-ffa450edef68