Specify that requests to set atime silently fail.
Specify that requests to set atime silently fail.

This will make it way easier to deal with setting mtime on OS X, where
the utimes(2) syscall doesn't appear to allow setting mtime but not
atime. This includes setting mtime in our own tests.

For #113.

mbrukman/gcsfuse,kahing/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse