Support the new parameters in do_clone(int argc, char** argv).

Now 'btrfs subvolume snapshot' takes not two but only at least two
parameters. Additionally, the help message is updated accordingly.

Signed-off-by: Andreas Philipp <philipp.andreas@gmail.com>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>

btrfs.c

@@ -50,8 +50,8 @@ static struct Command commands[] = {
 		avoid short commands different for the case only
 	*/
 	{ do_clone, 2,
-	  "subvolume snapshot", "<source> [<dest>/]<name>\n"
-		"Create a writable snapshot of the subvolume <source> with\n"
+	  "subvolume snapshot", "[-r] <source> [<dest>/]<name>\n"
+		"Create a writable/readonly snapshot of the subvolume <source> with\n"
 		"the name <name> in the <dest> directory.",
 	  NULL
 	},