MEDIUM: threads: detect incomplete CPU bindings

MEDIUM: threads: detect incomplete CPU bindings

It's very easy to mess up with some cpu-map directives and to leave
some thread unbound. Let's add a test that checks that either all
threads are bound or none are bound, but that we do not face the
intermediary situation where some are pinned and others are left
wandering around, possibly on the same CPUs as bound ones.

Note that this should not be backported, or maybe turned into a
notice only, as it appears that it will easily catch invalid
configs and that may break updates for some users.