Import the skein hashing algorithm, based on the threefish block cipher

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