Add I/O metrics on Mac

[alq666]

iostat -od 3 2 returns

       disk0        disk2        disk3 
 sps tps msps  sps tps msps  sps tps msps 
1722  54  0.0    1   0  0.0  764   7  0.0 
36036 150  0.0    0   0  0.0 35823 140  0.0

Only the last line matters.

The old-style iostat display (using -o) shows the following statistics:

• sps sectors transferred per second
• tps transfers per second
• msps average milliseconds per transaction

There is no clear separation between read and write operations, unification of metrics with linux is going to be difficult. If not possible because the metrics are inherently different, let's create new metrics.

[josePhoenix]

Looks like man 8 iostat on Mac doesn't reflect the actual command line options. Apparently iostat also uses the iostat [interval [count]] syntax (like Linux), but doesn't document it in the manpage or iostat -?... What I'm going to do is supply the -w and -c options for wait and count since those are in the manpage. (Still looking at how these map onto the Linux io stats.)

[josePhoenix]

Done in #530 / 6fd0ce8