Add L2ARC Compression

https://www.illumos.org/issues/3137
This issue ticket proposes adding transparent L2ARC compression to
ZFS. The algorithm is hard-coded to be the new LZ4 compression
algorithm, since L2ARC has very specific performance requirements
(high decompression speed).
The feature is controlled via a new secondarycachecompress property
on each dataset, allowing the user to selectively enable
compression only on datasets which can use it.

Patch dated 17/01/2013.