filter: allow selection of filter policy on startup

Added a new '--filter-policy=[POLICY]' option in the daemon, which
allows selecting between the supported filter policies. For now, only
two policies are defined:

  * default: the default policy used by ModemManager, where it tries
    to probe and detect as many modem ports as possible.

  * whitelist-only: only devices explicitly tagged via udev (with the
    ID_MM_DEVICE_PROCESS tag) will be probed and used.

src/main.c

@@ -87,6 +87,7 @@ bus_acquired_cb (GDBusConnection *connection,
     manager = mm_base_manager_new (connection,
                                    mm_context_get_test_plugin_dir (),
                                    !mm_context_get_no_auto_scan (),
+                                   mm_context_get_filter_policy (),
                                    mm_context_get_initial_kernel_events (),
                                    mm_context_get_test_enable (),
                                    &error);