variables hooks and bugs

[Yoshinori K. Okuji]

I changed the semantics of variable hooks, because I felt that the 
implementation by Marco was not sufficient to implement a hook for the root 
device. This change should not have any bad effect, because this feature was 
not used at all.

As you might notice this, I have been fixing several bugs, but the total 
number of known bugs does not decrease, because I find new bugs every time I 
try to fix existing bugs. This looks like endless.

Since I'm a bit tired of fixing bugs, I leave them to others, and I will do 
something more creative.

Okuji