Don't try to import a non-exported object in vcregress.pl

Don't try to import a non-exported object in vcregress.pl

Commit ca9e9b08e453 wrongly tried to import devnull from File::Spec, but
it's not exported, you just call the method via the class. This was
harmless until modern perls complained, so stop doing that.

Per buildfarm failures.

Backpatch 14 thru 16