Remove all ._FOO files before creating GAP release tarballs

[fingolfin]

As described in issue #112 I think we should reject package tarballs which contain macOS metadata in the form of ._FOO files. But in the meantime, we already ship such packages. Thus I think the release scripts should remove all such files before creating a release tarball. It's a good idea to be defensive here anyway, just in case a package update with such files manages to slip by our test, or if we introduce such files by some other way.

Shouldn't be too hard to implement, but (a) I don't know immediately which script is relevant, and (b) I hope we'll soon rewrite all these scripts from ground up anyway. So in the meantime I just want to make sure we won't forget.