sockets: plumb SO_PASSRIGHTS into *sockopt(2)

[kevans]

Summary

This is a little different than the others in that it's not valid for
anything but unix(4) sockets. New cases were added that jump into the
more standard case out of a light preference for not taking advantage of
case FALLTHROUGH with the additional logic- it doesn't scale very well
for new cases added that might be slightly special, so we might as well
just add the labels up-front.