nulldriver [PSARC/2008/674 Self Review]

[Chris Horne]

Darren
> I'd rather see a man page shipped even if it basically says "no user 
> serviceable parts inside - go away".
I have put a minimal nulldriver(7d) man page in the materials directory.
It communicates that having a devinfo node bound to nulldriver is normal,
at least in some circumstances, without going into details.
http://sac.eng/arc/PSARC/2008/674/materials/nulldriver.7d

This change implies a modification from 'private' to 'public'  for
'nulldriver' in the interface table.

-Chris