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/meson.build

@@ -12,6 +12,7 @@ executable (
         'manager.c', 'manager.h',
         'mm-iface.c', 'mm-iface.h',
         'suspend.c', 'suspend.h',
+        'udev.c', 'udev.h',
     ],
     dependencies : mgr_deps,
     install : true