Add ap_os_killpg as well as unixd_killpg and beosd_killpg.

Move the worker MPM to use ap_os_killpg in place of unixd_killpg as
this makes it more portable.

Ryan notes that really we should be using the POD mechanism to
control death of the child processes, so this is temporary until
we have that change in place.  Note added to STATUS to that effect.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92598 13f79535-47bb-0310-9956-ffa450edef68