tbf: Use LListHead instead of llist_pods

LListHead does basically the same like llist_pods, but more C++ish
and with type safety.

This commit turns the former list field of gprs_rlcmac_tbf into a
private field, provides accessors, moves the related code from
pcu_vty.c to pcu_vty_functions.cpp, and removes the llist_pods
type and related code.

Sponsored-by: On-Waves ehf
diff --git a/src/pcu_vty.c b/src/pcu_vty.c
index aa29b4c..ce9db29 100644
--- a/src/pcu_vty.c
+++ b/src/pcu_vty.c
@@ -775,19 +775,7 @@
       SHOW_STR "information about TBFs\n" "All TBFs\n")
 {
 	struct gprs_rlcmac_bts *bts = bts_main_data();
-	struct llist_head *tbf;
-
-	vty_out(vty, "UL TBFs%s", VTY_NEWLINE);
-	llist_for_each(tbf, &bts->ul_tbfs) {
-		tbf_print_vty_info(vty, tbf);
-	}
-
-	vty_out(vty, "%sDL TBFs%s", VTY_NEWLINE, VTY_NEWLINE);
-	llist_for_each(tbf, &bts->dl_tbfs) {
-		tbf_print_vty_info(vty, tbf);
-	}
-
-	return CMD_SUCCESS;
+	return pcu_vty_show_tbf_all(vty, bts);
 }
 
 DEFUN(show_ms_all,