Enhance the file filtering capabilities.
We now allow filtering through either pipes or pairs of temporary
files. The latter appear to be mandatory for use on Windows.
port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export