Merge r1561265 from trunk:

Because of Darwin/OSX, we need to worry about
the pathlength, which is much less than 255. So use
the method from posix sems, which we've used for
years!

Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1561266 13f79535-47bb-0310-9956-ffa450edef68