lib: fix command-line flag of `info` command
lib: fix stupid bug in `Host.Ssh`
build,lib: use `dbm` for `Ketrew_database`

Dbm is mostly standard but does not provide transactions, so there is a
“quick & dirty” implementation of a rollback in `Database.act`.
Sqlite seems to be broken for MacOSX (either fails to install, or later
fails to link with Lwt executatbles).
lib: change default filename for the database
lib: fix display in `Database.log_error`
lib: add function to close database from `State`
lib: move `with_state` to `Ketrew_state`
lib: remove `create`/`release` form `State`'s API

Now one has to use `with_state` only.
lib: remove useless Atdgen definitions
lib: add notion of “archived” targets
lib: add function `State.archive_target`
lib: fix `State.with_state` (handle all errors)
lib: add the `archive` sub-commmand