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.

Better errno handling

the version of the system and the version of services

create, restore, remove and list are available.
These subcommand do not affect the state of a running services.

Add islog internal key to quickly is the resolve file concern the logger itself.
Add the rversion internal field. This field will be used to know the previous
version of the resolve file and possibly make an
automatic migration of the db to the new format.

Typo fix

Follow skarnet changes using stralloc and genalloc.

In addition to the string "@I" as identifier to replace by the name of instance service,
the following identifier are now recognize :
@I, @U, @u, @G, @g, @H, @S, @R. see doc/66-identifier.md for furthers information.