dylan update: prompt if registry is under VC

[cgay]

If you run dylan update in the dylan-tool directory itself, it writes platform-specific registry files that shadow the generic registry entries that are under version control. Either it should prompt the user "ok? (yes/no)" or just abort with a message.

[cgay]

Actually, I think Deft can write registry files in "_registry" instead without a problem, and just make sure that directory is first in the registry list when invoking dylan-compiler.