dev: Don't fail to start if we cannot open the dev plugin path

We accidently returned stdin fd in this case - return -1 instead.

src/dev.c

@@ -150,7 +150,7 @@ dev_start1(struct dhcpcd_ctx *ctx, const struct dev_dhcpcd *dev_dhcpcd)
 	dp = opendir(DEVDIR);
 	if (dp == NULL) {
 		logdebug("dev: %s", DEVDIR);
-		return 0;
+		return -1;
 	}
 
 	r = 0;