When opening a trace file, open O_NONBLOCK so that ktrace doesn't stall if the target is a fifo.  After opening a trace file, check that it is a regular file, and if not, return an error.
When opening a trace file, open O_NONBLOCK so that ktrace doesn't stall
if the target is a fifo.  After opening a trace file, check that it is a
regular file, and if not, return an error.

MFC after:	3 days
Reported by:	kris
PR:		94278

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