plugin: preserve stderr for plugins.

Now we've updated ccan/pipecmd, we can use pipecmd_preserve to
preserve stderr for plugins so we see their error spew.

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

lightningd/plugin.c

@@ -785,7 +785,7 @@ void plugins_init(struct plugins *plugins, const char *dev_plugin_debug)
 		cmd[0] = p->cmd;
 		if (debug)
 			cmd[1] = "--debugger";
-		p->pid = pipecmdarr(&stdin, &stdout, NULL, cmd);
+		p->pid = pipecmdarr(&stdin, &stdout, &pipecmd_preserve, cmd);
 
 		if (p->pid == -1)
 			fatal("error starting plugin '%s': %s", p->cmd,