Rename hiercopy entry to copyfrom.

The dependencies was displayed at first level of identation
making confusing the comprehension of the list.
The dependencies whatever the associated tree to that dependency is displayed
at depth level above 1.

Try to be explicit at redirection doc.

Provide new keyword stdin, stdout and stderr at -o options.
Decrease number of logger line to display to 10.

See comment line 290 at parse_bracket function.

Prevent user to remove service inside a module.

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

Remove deprecated options 'n', 'a', 'd', 'C', 'S'.

at 66-execute invocation.
Default is 2.

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