grid: Keep using the previous socket path name if it still exists
In 58fbaabf (svcbackend/standalone: use shorter names for supervisor sockets , 2020-07-17) we changed the socket path from supervisord.sock to a shorter sv.sock, but this caused issues when updating running slapos node instances, because this was trying to start a new supervisord process using the new socket path, while the old one was still running.
To keep services running after upgrade, introduce a compatibility layer in this socket path - if we still have a supervisord running with a socket at this previous path, keep using this old socket name.