Add a warning about Giant Locked devices

Add a warning when a device registers with devfs and requests
D_NEEDGIANT. The warning says the device will go away before
13.0. This is needed to flush out the devices in the tree that are
still Giant locked. This warning, or some variant of it, should have
gone into the tree a long time ago...

The intention is to require all devices be converted to not use
automatic giant in this way, or remove any such devices that remain
that we don't have the hardware to test a conversion of.

kbd so far is the only device that can't leave the tree, yet needs
something sensible done to avoid the auto giant lock (even if it is
just doing the wrapping itself). There may be others added to this
list... Any discussions of this topic will take place on arch@.