BUG/MINOR: threads: set threads_idle and threads_harmless even with no threads

BUG/MINOR: threads: set threads_idle and threads_harmless even with no threads

Some signal handlers rely on these to decide about the level of detail to
provide in dumps, so let's properly fill the info about entering/leaving
idle. Note that for consistency with other tests we're using bitops with
t->ltid_bit, while we could simply assign 0/1 to the fields. But it makes
the code more readable and the whole difference is only 88 bytes on a 3MB
executable.

This bug is not important, and while older versions are likely affected
as well, it's not worth taking the risk to backport this in case it would
wake up an obscure bug.