- Some refactoring of `vm-auto-archive-messages' and
`vm-virtual-auto-archive-messgaes' (the latter in vm-avirtual.el).
- Added a new variable `vm-virtual-make-up-auto-folder-names' controlling
whether virtual folder names are used as auto-folders.
Made vm-interactive-p into a macro.  The attempt to define it using fset
doesn't work.