Relicense zfs.gentoo.in from GPLv2 to 2-clause BSD

Relicense zfs.gentoo.in from GPLv2 to 2-clause BSD

As the Gentoo sys-fs/zfs maintainer, I receive license compatibility
questions and at times, those questions can be harassing. I feel that
the presence of the GPL in Gentoo's package metadata promotes such
questions.  zfs.gentoo.in is the only GPLv2 licensed file in ZFS, so I
have taken the liberty of contacting all contributors to this file to
request permission to relicense it.

All of the contributors to this file have agreed to relicense it under
the 2-clause BSD license. I have added their Signed-offs to this commit,
in order of first contribution. Thank you everyone for being so
understanding.

Signed-off-by: devsk <devsku@gmail.com>
Signed-off-by: Alexey Shvetsov <alexxy@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrew Tselischev <andrewtselischev@gmail.com>
Signed-off-by: Zachary Bedell <zac@thebedells.org>
Signed-off-by: Gunnar Beutner <gunnar@beutner.name>
Signed-off-by: Kyle Fuller <inbox@kylefuller.co.uk>
Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Closes #819