Giulio Eulisse reported that on OSX, bad free space numbers were being
shown. It thought he had negative free space.
While the documentation is not clear, especially across OS's, it seems
likely that statfs uses unsigned long. It doesn't make sense for any
numbers to be negative.
Tested on Debian kfreebsd-amd64. The BSD #includes worked. Both statfs64
and statfs worked. Using statfs to keep the same as general freebsd, and
because I didn't try it on 32 bit.
At least on OSX 10.6.6 it appears to have the same defintions as
FreeBSD. The build process doesn't complain and the code is enabled,
this needs to be tested and checked more.
