User registry hive

[Corinna Vinschen]

I have just for fun implemented to load the users registry hive
in spawn_guts in case of CreateProcessAsUser. It works fine.

Are we interested in doing that? 

That slows down things a bit when changing the user context but
that doesn't happen that often.

Still unresolved is the problem if somebody tries to change
something in the own hive, say mount points. Because there's
currently no mechanism to save the hive back again, it's new
content is discarded on reboot.

Corinna