unset MAGIC on darwin

different platforms have different versions of file(1) installed.
file(1) reads compiled magic files, which are not compatible with
earlier versions of itself

until we think of a way to compile the magic files in etc/pkg/file/
(perhaps at installation time via a script in etc/scripts/), we
can just unset the variable here.
Merge branch 'master' of github.com:gjvc/etc