The double compilation of the kernel sources in libmd and libcrypt is
baffling, but add yet another define hack to prevent duplicate symbols.
Add documentation and SHA2-224 test cases to libmd.
Integrate with the md5(1) command, document, and add more test cases;
self-tests pass.
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
