TBF: add dedicated log categories

Previously all TBF-related events were logged as part of DRLCMAC which
is too broad to make it practically useful due to excessive amount of
log messages generated. Introduce dedicated log categories for
TBF-related events. Adjust test output as necessary.

Change-Id: I64d660e5971263d5c63d2ba95d50625c16a594aa
diff --git a/src/gprs_debug.h b/src/gprs_debug.h
index 747465b..4c57633 100644
--- a/src/gprs_debug.h
+++ b/src/gprs_debug.h
@@ -37,6 +37,9 @@
 	DRLCMACUL,
 	DRLCMACSCHED,
 	DRLCMACMEAS,
+	DTBF,
+	DTBFDL,
+	DTBFUL,
 	DNS,
 	DBSSGP,
 	DPCU,