MFC r271747:

• 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).