A joining module's own directories belong to its own group, and its tools run in the
tool runner, which could not read them. An identity may now name the tool runner's
account: the engine puts it in mesh-<module> as it puts it in a library's group,
says the change as identity:<module> for the runner's unit to restart once, takes
it back when no identity names it, and refuses an account that is missing or root's.
The engine states contract 7.