mpm_unix: Add ap_mpm_podx_* implementation to avoid code duplication

and align w/ trunk.

trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1409214
             http://svn.apache.org/viewvc?view=revision&revision=1410004
2.4.x patch: http://people.apache.org/~jim/patches/httpd-2.4-podx-v2.patch

Submitted by: jim
Reviewed by: rjung, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1542557 13f79535-47bb-0310-9956-ffa450edef68