Defined more of the file system interface.
Defined more of the file system interface.

mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,jacobsa/ratelimit,kahing/gcsfuse,jacobsa/timeutil,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,BanzaiMan/gcsfuse