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.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2020 Arnaud Ferraris <arnaud.ferraris@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include "udev.h"
+
+#include <string.h>
+
+static void udev_event_cb(GUdevClient *client, gchar *action, GUdevDevice *device, gpointer data)
+{
+    struct EG25Manager *manager = data;
+
+    if (strcmp(action, "unbind") != 0 || manager->modem_state == EG25_STATE_RESETTING)
+        return;
+
+    if (strncmp(g_udev_device_get_name(device), manager->modem_usb_id, strlen(manager->modem_usb_id)) == 0 &&
+        manager->reset_timer == 0) {
+        g_message("Lost modem, resetting...");
+        modem_reset(manager);
+    }
+}
+
+void udev_init (struct EG25Manager *manager)
+{
+    const char * const subsystems[] = { "usb", NULL };
+
+    manager->udev = g_udev_client_new(subsystems);
+    g_signal_connect(manager->udev, "uevent", G_CALLBACK(udev_event_cb), manager);
+
+    return;
+}
+
+void udev_destroy (struct EG25Manager *manager)
+{
+    if (manager->udev) {
+        g_object_unref(manager->udev);
+        manager->udev = NULL;
+    }
+}