Relicense under the (Mirage standard) ISC license

To upstream the recent changes to the [mirage/ocaml-tar] project, the license should be changed to the OpenBSD-style ISC license. See [mirage/ocaml-tar#5]

From the code history the copyright holders are
- Citrix
- Thomas @samoht
- David @dra27

@dra27 -- are you ok with this?