Cannot run psql in postgresql 9.5
The permissions on "/etc/ssl/private" should be "drwx--x---" (aka "0710"), which should have been the permissions it came with.  I don't know it got changed.  You could restore it with:

sudo chmod 0710 /etc/ssl/private/


Although maybe there is also a way to do it with your package manager.

You don't need a server.key file in "/var/lib/postgresql/9.5/main", because postgresql is configured to look for it in "/etc/ssl/private/ssl-cert-snakeoil.key" instead.  You can make or get your own cert and put in "/var/lib/postgresql/9.5/main", but then you will have to change the configuration to look for it there.