src: add udev watcher to improve modem recovery

Most of the modem issues follow a (incomplete) USB device reset. Instead 
of relying solely on the existing timer, this patch adds a udev monitor 
which resets the modem as soon as its associated USB device is reset, 
which greatly improves recovery time.

src/udev.h

@@ -0,0 +1,12 @@
+/*
+ * Copyright (C) 2020 Arnaud Ferraris <arnaud.ferraris@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "manager.h"
+
+void udev_init (struct EG25Manager *data);
+void udev_destroy (struct EG25Manager *data);