Follow up to r1363557:

Refactor with some existing code to slightly optimize thread tracking.

Fix an implementation problem affecting statically linked MPMs which 
would break the ability to fail restart if a resource shortage was
found immediately.

Found by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1364601 13f79535-47bb-0310-9956-ffa450edef68