This should better stress memclr.

I misunderstood what was being zeroed. Only the header is.

There are a few changes to tests and documentation to reflect changes to
osxfuse behavior. All of them should probably be seen as backwards
compatible. The tests now test the osxfuse 3 behavior, and for this
reason we should consider killing osxfuse 2 support after some time.

Closes #18.
Closes #19.

Tested on OS X 10.11.6 (15G1004) with osxfuse 3.5.2.

There's no need to complicate the API. If osxfuse makes a
backwards-incompatible change again, we can add support for it.

Also supports custom bundled OSXFUSE installs.

To aid in debugging GoogleCloudPlatform/gcsfuse#170.

For googlecloudplatform/gcsfuse#161.

In particular TestNonEmptyMountPoint, which is currently failing on
Linux due to the poor error handling we inherited from bazil.org/fuse.

I had forgotten that we explicitly handle the "mount point doesn't
exist" case, so that wasn't tickling what I had hoped in fusermount.

It appears osxfuse is happy to mount over a non-empty directory.

This makes it easier to write integration tests for atime, ctime, and
mtime on both darwin and linux.

For googlecloudplatform/gcsfuse#137.

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.