Merge r745763 from trunk:

Enable unix domain (AF_UNIX) sockets if supported by the OS
Submitted by: mturk
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.6.x@1590714 13f79535-47bb-0310-9956-ffa450edef68