Read PDU session configuration and request

Use the new PDU session configuration functionality of the parent
commit. It is now possible to request multiple PDU sessions, and to
request specific PDU session types. While the new uicc0.pdu_sessions is
preferred, it is still possible to use the "legacy"
uicc0.dnn/nssai_sst/nssai_sd.

The old parameters --default-pdu-id and --extra-pdu-id are not allowed
anymore, as it is possible to manually define this in the UE
configuration. Correspondingly, fix the use of those options, where
applicable.

Note: since the default_pdu_id is changed to -1, all interface names
will have pX to interface names appended (oaitun_ue1pX). The next commit
will correct this.

executables/nr-uesoftmodem.c

@@ -181,10 +181,8 @@ static void get_options(configmodule_interface_t *cfg)
   config_get(cfg, cmdline_params, numparams, NULL);
   if (nrUE_params.vcdflag > 0)
     ouput_vcd = 1;
-  AssertFatal(nrUE_params.extra_pdu_id != get_softmodem_params()->default_pdu_session_id,
-              "Default PDU ID (%d) and Extra PDU ID (%d) must be different!\n",
-              get_softmodem_params()->default_pdu_session_id,
-              nrUE_params.extra_pdu_id);
+  AssertFatal(nrUE_params.extra_pdu_id == -1,
+              "Add additional PDU sessions in uicc.pdu_sessions array instead\n");
 }
 
 // set PHY vars from command line