Add stats: pcu.bts.N.pdch.occupied.gprs/egprs

Add stats needed for performance measurements in
3GPP TS 52.402 ยง B.2.1.54-55.

Split m_num_tbfs to count GPRS and EGPRS TBFs separately. Move the code
that updates m_num_tbfs and sets the PDCH_OCCUPIED stats to a separate
function, as it's mostly the same in the TBF attach and detach.

Related: SYS#4878
Change-Id: I0c0a1121b4ae5f031782e7e63a0c28eb0b6c8b42
diff --git a/src/bts.h b/src/bts.h
index a6e7150..d9a86eb 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -184,6 +184,8 @@
 	STAT_MS_PRESENT,
 	STAT_PDCH_AVAILABLE,
 	STAT_PDCH_OCCUPIED,
+	STAT_PDCH_OCCUPIED_GPRS,
+	STAT_PDCH_OCCUPIED_EGPRS,
 };
 
 /* RACH.ind parameters (to be parsed) */