I just ran in to this problem affecting my ekeyd installation (just reported as #852380). Unix domain sockets are available, but the API changed in the new version.
Instead of calling socket.unix() to create a socket, you now need to call socket.unix.tcp(). Note that there is another API change coming that changes this to socket.unix.stream(). (This change is not yet in Debian. It's upstream commit 3a33c37b; https://github.com/diegonehab/luasocket/commit/3a33c37b9ce852d0b3531e82c6ffdb47bb937f0a.)
