lightningd: remove deprecated_apis global, put into lightningd.

We usually have access to `ld`, so avoid the global.

The only place generic code needs it is for the json command struct,
and that already has accessors: add one for libplugin and lightningd
to tell it if deprecated apis are OK.

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

plugins/libplugin.c

@@ -500,6 +500,11 @@ bool command_usage_only(const struct command *cmd)
 	return cmd->usage_only;
 }
 
+bool command_deprecated_apis(const struct command *cmd)
+{
+	return deprecated_apis;
+}
+
 /* FIXME: would be good to support this! */
 bool command_check_only(const struct command *cmd)
 {