Implemented most of dir.Mknod.
Implemented most of dir.Mknod.

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