disk.usage reports mangled information on MacOS-10.8.x

[netllama]

The disk.usage function doesn't seem to handle MacOS-10.8.x correctly,
and reports mangled information for both the mount points and their
usage statistics. I'm seeing this behavior with 0.15.1.

In the output below, cuda-mac64-build14 is a 10.8.3 system, and cuda-mac-test0 is a 10.7.3 system:

cuda-mac64-build14:
----------
100%:
----------
1K-blocks:
-static
available:
0
capacity:
0
filesystem:
map
used:
0
23954090:
----------
1K-blocks:
1951845952
available:
1760213216
capacity:
10%
filesystem:
/dev/disk0s2
used:
191120736
624:
----------
1K-blocks:
360
available:
0
capacity:
100%
filesystem:
devfs
used:
360
cuda-mac-test0:
----------
/:
----------
1K-blocks:
311909984
available:
97990280
capacity:
69%
filesystem:
/dev/disk0s2
used:
213407704
/dev:
----------
1K-blocks:
216
available:
0
capacity:
100%
filesystem:
devfs
used:
216
100%:
----------
1K-blocks:
-static
available:
0
capacity:
0
filesystem:
map
used:
0

cuda-mac64-build14:~ lfriedman$ df -k
Filesystem 1024-blocks Used Available Capacity iused ifree
%iused Mounted on
/dev/disk0s2 975922976 95560392 880106584 10% 23954096 220026646 10% /
devfs 180 180 0 100% 624 0
100% /dev
cuda-mac-test0:~ lfriedman$ df -k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/disk0s2 155954992 106703856 48995136 69% /
devfs 108 108 0 100% /dev

[basepi]

So as we discussed, I think we just need to add disk.usage logic specific to OSX.

I've added to the current milestone.