Set subtype to gcsfuse

This allows identifying mounts as "fuse.gcsfuse" instead of only
"fuse" on Linux.  This is useful for pjdfstest so it can exclude tests
that are not supported.  References #590.

mount.go

@@ -130,6 +130,7 @@ be interacting with the file system.`)
 	status.Printf("Mounting file system %q...", fsName)
 	mountCfg := &fuse.MountConfig{
 		FSName:      fsName,
+		Subtype:     "gcsfuse",
 		VolumeName:  "gcsfuse",
 		Options:     flags.MountOptions,
 		ErrorLogger: logger.NewError("fuse: "),