When starting up threads in taskqueue_start_threads create them
stopped before adjusting their priority and setting them on the run
q so they cannot race for resources (pointed out by njl).

While here add a console printf on thread create fails; otherwise
noone may notice (e.g. return value is always 0 and caller has no
way to verify).

Reviewed by: jhb, scottl
MFC after: 2 weeks