zfs: Port lz4 compression algorithm from FreeBSD

OSv port details:
- Discarded manpage changes.
- lz4 license was added to the licenses directory.
- Addressed some conflicts in zfs/zfs_ioctl.c.
- Add unused attributed to a few functions in zfs/lz4.c which are
actually unused.

 * Illumos zfs issue #3035 [1] LZ4 compression support in ZFS.

LZ4 is a new high-speed BSD-licensed compression algorithm created
by Yann Collet that delivers very high compression and decompression
performance compared to lzjb (>50% faster on compression, >80% faster
on decompression and around 3x faster on compression of incompressible
data), while giving better compression ratio [1].

FreeBSD commit hash: c6d9dc1

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>