ms: Get the set of slots currently active

This commits adds methods to GprsMs and gprs_rlcmac_tbf to retrieve
the slots that are actively used.

Sponsored-by: On-Waves ehf
diff --git a/src/tbf.h b/src/tbf.h
index 82ade4e..f6f13f7 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -158,6 +158,8 @@
 	const gprs_llc_queue *llc_queue() const;
 
 	time_t created_ts() const;
+	uint8_t dl_slots() const;
+	uint8_t ul_slots() const;
 
 	/* attempt to make things a bit more fair */
 	void rotate_in_list();