I'm aware of how I can solve it. I wanted to understand why after installing the pg 9.6 packages suddenly psql tries to access the socket on /var/run/postgresql. Does the libpq default unix socket is changed between those two versions ? (9.6,9.2)
It is not a version issue, but a packaging issue. Different systems have different conventions on where sockets should go, and the packager imposes their opinion on the things they package.