In linux, if the first byte of sun_path in struct sockaddr_un is \0, the socket
exists in an abstract namespace not connected to the filesystem, and the name of
the socket is given by the other UNIX_PATH_MAX-1 bytes in sun_path.

xtrans has issues with creating sockets in /tmp, including some well known
security problems (bug #951 for example).  Some projects are working towards a
per-user /tmp, but the server sockets have to be globally accessible.  Creating
them in the abstract namespace would keep them both globally visible and out of
the filesystem.

Abstract sockets otherwise behave like normal PF_UNIX sockets so no security
problems are introduced by this.  For compatibility xtrans should try both
filesystem and abstract sockets.