Destination field is replaced by StdXXX key field.
StdXXX s6log type accept now path for a specific destination.

Allow to use an existing snapshot at upgrade time

For the peace of mind of user and dev, 66 handles now
on its own the migration of the different components
of the 66 ecosystem and user or distributor have nothing to do at upgrade time.
The migration process do not affect running services.
Its also make a snapshot before making the migration. So, user
can retrieve easily the previous working state of the system in case
of crash of the migration process.