While I was at it, added tests for fsname. I can't figure out how to
test the volume name.

For GoogleCloudPlatform/gcsfuse#125.

File systems must now handle StatFSOp in order to mount correctly on
OS X.

For GoogleCloudPlatform/gcsfuse#125.

statfs(2) is more involved than I expected, so this calls for its own
sample file system to test in a more focused manner.

This may bring significant performance improvements, but also comes with
some caveats. See the notes on MountConfig.DisableWritebackCaching for
details.