- Use NULL instead of 0 for fpcurthread.
- Note the quirk with the interrupt enabled state of the dna handler.
- Use just panic() instead of printf() and panic(). Print tid instead
of pid, the fpu state is per-thread.
MFC r271924:
Update and clarify comments. Remove the useless counter for impossible, but
seen in wild situation (on buggy hypervisors).
24883994ff