param: add ok flag to struct command

Callers to param() can now optionally set a flag to see if command_fail was
called.

This is necessary because the `cmd` is freed in case of failure.

I spent a bit of time trying to extend the lifetime of the `cmd` to the end
of parse_request(), but the destructors still needed to be called when they
were, and it was getting ugly.  So I took this minimal approach.

Signed-off-by: Mark Beckwith <wythe@intrig.com>

lightningd/jsonrpc.h

@@ -37,6 +37,7 @@ struct command {
 	enum command_mode mode;
 	/* This is created if mode is CMD_USAGE */
 	const char *usage;
+	bool *ok;
 };
 
 struct json_connection {