box: fix and also refactor privilege_resolve

Privileges passed to box.schema.user.grant are resolved in quite a
naïve manner: we do not check that all of them have been resolved.

Also, the privilege_resolve function itself is poorly designed (instead
of generic string processing, we have a bunch of if's): extract parsing
of the privileges string into privilege_parse and process it
through a generic FSA.

Closes #6199