Merge branch 'arc/exclusions-in-package-file'

Merge branch 'arc/exclusions-in-package-file'

This branch pulls in some of the later changes from rmanfredi/dist#14.
Raphaël suggested that the `.package` file be permitted to name the file
used for the `-X` option, and that `packinit` prompt for it. Those changes
have now been applied to our local copy of the (generated) `bin/mconfig`
file, and I've regenerated our `.package` file with the relevant setting.
Finally, our README no longer recommends the option, as it's no longer
needed.