Add ZFS compressed ARC stats to top(1)

Provides:
amount of compressed data
logical size of compressed data (how much it would have taken uncompressed)
compression ratio (logical size : total ARC size)
Overhead (space consumed for compression headers)

Example output:
ARC: 31G Total, 18G MFU, 9067M MRU, 2236K Anon, 615M Header, 2947M Other

25G Compressed, 54G Uncompressed, 1.76:1 Ratio, 2265M Overhead

Reviewed by: jpaetzel, smh, imp, jhb (previous version)
MFC after: 2 week
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D9829