Added support for Posix semaphore-based mutex locking (AcceptMutex posixsem).

It's between pthread and sysvsem in the DEFAULT priority ranking. This
makes it the new default for Darwin, and adds support for it for other
platforms as well (like Solaris).
PR:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94434 13f79535-47bb-0310-9956-ffa450edef68