common: new function command_log to log something about a specific command.

Needs implementations for lightningd and libplugin, since they both use this
infrastructure.

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

common/json_command.h

@@ -6,6 +6,7 @@
 #include <ccan/compiler/compiler.h>
 #include <common/json_parse.h>
 #include <common/jsonrpc_errors.h>
+#include <common/status_levels.h>
 
 struct command;
 struct command_result;
@@ -33,6 +34,11 @@ command_fail_badparam(struct command *cmd,
 			    json_tok_full(buffer, tok));
 }
 
+/* Do some logging (complaining!) about this command misuse */
+void command_log(struct command *cmd, enum log_level level,
+		 const char *fmt, ...)
+	PRINTF_FMT(3, 4);
+
 /* Also caller supplied: is this invoked simply to get usage? */
 bool command_usage_only(const struct command *cmd);