Tools like du and ls -l use this value to calculate the space used.
Force block size to 512 bytes for simplicity and compatibility with
Mac OS X which uses an older FUSE.

On OS X we can't set block size above 2^17, but we want to encourage
users to send large reads and writes.

For GoogleCloudPlatform/gcsfuse#125.

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.