param: abstract 'struct command' so param doesn't need to access it.

I want to use param functions in plugins, and they don't have struct
command.

I had to use a special arg to param() for check to flag it as allowing
extra parameters, rather than adding a one-use accessor.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/json_command.h

@@ -0,0 +1,24 @@
+/* These functions must be supplied by any binary linking with common/param
+ * so it can fail commands. */
+#ifndef LIGHTNING_COMMON_JSON_COMMAND_H
+#define LIGHTNING_COMMON_JSON_COMMAND_H
+#include "config.h"
+#include <ccan/compiler/compiler.h>
+#include <stdbool.h>
+
+struct command;
+
+/* Caller supplied this: param assumes it can call it. */
+void PRINTF_FMT(3, 4) command_fail(struct command *cmd, int code,
+				   const char *fmt, ...);
+
+/* Also caller supplied: is this invoked simply to get usage? */
+bool command_usage_only(const struct command *cmd);
+
+/* If so, this is called. */
+void command_set_usage(struct command *cmd, const char *usage);
+
+/* Also caller supplied: is this invoked simply to check parameters? */
+bool command_check_only(const struct command *cmd);
+
+#endif /* LIGHTNING_COMMON_JSON_COMMAND_H */