sierra-legacy: implement connection status monitoring with !SCACT?

Devices like the Netgear AC313U require explicit context monitoring,
otherwise the device may end up disconnected internally and MM would
still think that the connection is ongoing.

src/mm-modem-helpers.c

@@ -1554,7 +1554,7 @@ mm_3gpp_pdp_context_active_list_free (GList *pdp_active_list)
     g_list_free_full (pdp_active_list, (GDestroyNotify) mm_3gpp_pdp_context_active_free);
 }
 
-static gint
+gint
 mm_3gpp_pdp_context_active_cmp (MM3gppPdpContextActive *a,
                                 MM3gppPdpContextActive *b)
 {

src/mm-modem-helpers.h

@@ -181,6 +181,8 @@ typedef struct {
     gboolean active;
 } MM3gppPdpContextActive;
 void mm_3gpp_pdp_context_active_list_free (GList *pdp_active_list);
+gint mm_3gpp_pdp_context_active_cmp (MM3gppPdpContextActive *a,
+                                     MM3gppPdpContextActive *b);
 GList *mm_3gpp_parse_cgact_read_response (const gchar *reply,
                                           GError **error);