Convert ap_proc_t to a complete type. This lets us access the pid directly.

Only the prefork MPM has been ported so far, the rest of the Unix MPM's are
coming later today.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85279 13f79535-47bb-0310-9956-ffa450edef68