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.