Default socketdir (`/var/run`) is restricted area

Hi,

The default directory for file system sockets (`/var/run`) is generally writable by root only, making file system sockets usable by privileged applications only. On the other hand abstract sockets (used by default on Linux) have no access control whatsoever. Looks like the default socket file directory should be changed to a world-writable one like `/tmp` to achieve similar behavior on Linux and BSDs for example. This have already come up a couple of times: https://github.com/ClusterLabs/libqb/pull/248#issuecomment-348992701, https://github.com/ClusterLabs/libqb/issues/222#issuecomment-355989054.
Is there a good reason to stay with the restricted default? Or not to introduce some means of changing the socket directory from the application (for example a new API call)?

Thanks,
Feri.