Use "allow_other" instead of "allow_user".

Maybe I'm missing something, but my version of FUSE does not seem
to document "allow_user" at all.

filejoinfs.c

@@ -280,7 +280,7 @@ int main(int argc, char *argv[]) {
 	fuse_argv[0] = argv[0];
 	fuse_argv[1] = mountpoint_file;
 	fuse_argv[2] = "-o";
-	fuse_argv[3] = "nonempty,allow_user,direct_io";
+	fuse_argv[3] = "nonempty,allow_other,direct_io";
 	fuse_argv[4]  = 0;
 
 	ret = fuse_main(4, fuse_argv, &concatfs_op, NULL);