Import the skein hashing algorithm, based on the threefish block cipher
Connect it to userland (libmd, libcrypt, sbin/md5) and kernel (crypto.ko)
Support for skein as a ZFS checksum algorithm was introduced in r289422
but is disconnected because FreeBSD lacked a Skein implementation.
A further commit will enable it in ZFS.
Reviewed by: cem
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D6166