using multiple VC backends in the same directory

[Dan Nicolaescu]

Working with multiple VC backends in the same directory should work a
bit better now.
With C-u C-x v d it is possible to specify what VC backend to use.

If one of the backends is RCS, some VC commands might get confused
because this Todo item:

;; - vc-expand-dirs should take a backend parameter and only look for
;;   files managed by that backend.

is still looking for someone interested to implement it.