Create pmain pool and run modules' child_init hooks when entering

Create pmain pool and run modules' child_init hooks when entering
ap_mpm_run(), then destroy pmain when exiting ap_mpm_run().
The expected call to ap_run_child_init() appears to have been removed
in r89640.  However, that call should presumably still be made once per
process, as in other single-process MPMs like the netware MPM.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@491922 13f79535-47bb-0310-9956-ffa450edef68