lightningd: implement checkrune command.

This extracts the core checking functionality for a rune, so they can
easily be used more widely than just commando.

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

common/jsonrpc_errors.h

@@ -111,6 +111,11 @@ enum jsonrpc_errcode {
 	/* Errors from delforward command */
 	DELFORWARD_NOT_FOUND = 1401,
 
+	/* Errors from runes */
+	RUNE_NOT_AUTHORIZED = 1501,
+	RUNE_NOT_PERMITTED = 1502,
+	RUNE_BLACKLISTED = 1503,
+
 	/* Errors from wait* commands */
 	WAIT_TIMEOUT = 2000,
 };