Eliminate most mentions of "special"

Previously, the "spare" vdev type was described as "A special
pseudo-vdev which...".  I wanted to eliminate the word "special" from
that, now that the allocation_classes feature exists and there is such a
thing as a "special vdev".  I ended up eliminating almost all instances
of the word "special" that are not referencing the allocation_classes
feature.

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Richard Laager <[email protected]>
Closes #8626

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