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/mm-context.h

@@ -19,6 +19,8 @@
 #include <config.h>
 #include <glib.h>
 
+#include "mm-filter.h"
+
 #if !defined(MM_DIST_VERSION)
 # define MM_DIST_VERSION VERSION
 #endif
@@ -30,6 +32,9 @@ gboolean     mm_context_get_debug                 (void);
 const gchar *mm_context_get_initial_kernel_events (void);
 gboolean     mm_context_get_no_auto_scan          (void);
 
+/* Filter support */
+MMFilterRule mm_context_get_filter_policy (void);
+
 /* Logging support */
 const gchar *mm_context_get_log_level               (void);
 const gchar *mm_context_get_log_file                (void);