box: add `space_id_is_system` helper

In some cases we don't have the whole space struct, but we want to
determine whether the provided space identifier corresponds to a system
space — add a `space_id_is_system` helper and refactor `space_is_system` to
reuse it.

Needed for #7974

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring

(cherry picked from commit 928e5733)