Cmd::Unit: made set_message() protected; added commit_with().
Cmd::Data::RenameMetaField: restructured.¹

1: Some validation issues.
Cmd::Unit: removed operator!().¹

1: Bool conversion should not be implicit, even under a `!`.
ok() and bad() are explicit; better.
Cmd: added enum Cmd::Result; reworked Unit result value.
Cmd: satisfied the Doxygen Daemon.
Cmd::Data: corrected exception reports.
Cmd/Data: added command Cmd::Data::RemoveMetaField.