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/manager.h

@@ -8,6 +8,7 @@
 
 #include <glib.h>
 #include <gpiod.h>
+#include <gudev/gudev.h>
 #include <libmm-glib.h>
 
 enum EG25State {
@@ -44,6 +45,7 @@ struct EG25Manager {
     int suspend_inhibit_fd;
     guint suspend_source;
 
+    GUdevClient *udev;
 
     struct gpiod_chip *gpiochip[2];
     struct gpiod_line *gpio_out[5];