lib: create mmap files in socket directory

Currently the mmap files are created in LOCALSTATEDIR/run on non-Linux
platforms which can be problematic with pacemaker since it spawns processes
as the hacluster user, which by default doesn't have write permissions to
it. Using --with-socket-dir partially fixes the problem by allowing the
unix sockets to reside somewhere else but not the mmap files and this
patch puts them in the same directory.