api,modem: new 'SimSlots' and 'PrimarySimSlot' properties

The 'SimSlots' property exposes an array of SIM object paths, with one
array item for each available SIM slot in the system. If a valid SIM
card is found in a given slot, the path of the SIM object will be
exposed in the array item; if no valid SIM card is found, the empty
object path ("/") will be exposed instead.

The 'PrimarySimSlot' property exposes which of the SIM slots available
in the system is the one configured as being primary. In a Multi-SIM
Single-Standby setup, the primary slot will be the one corresponding
to the single active SIM in the system. In a Multi-SIM Multi-Standby
setup, the primary slot will be the one configured to act as primary
(e.g. the one that will be used for the data connection) among all the
active SIM cards found.

cli/mmcli-modem.c

@@ -475,6 +475,8 @@ print_modem_info (void)
 
     sim_path = mm_modem_get_sim_path (ctx->modem);
     mmcli_output_string (MMC_F_SIM_PATH, g_strcmp0 (sim_path, "/") != 0 ? sim_path : NULL);
+    mmcli_output_sim_slots (mm_modem_dup_sim_slot_paths (ctx->modem),
+                            mm_modem_get_primary_sim_slot (ctx->modem));
 
     bearer_paths = (const gchar **) mm_modem_get_bearer_paths (ctx->modem);
     mmcli_output_string_array (MMC_F_BEARER_PATHS, (bearer_paths && bearer_paths[0]) ? bearer_paths : NULL, TRUE);