fix: DLGSUP logging category "unusable"

All DL* categories are typically negative, but DLGSUP isn't, and it's also not
in libosmocore's internal_cat array.
See: 3b6fb0880c3ab1e23a3d7d738d073b00c2a794c2

This means that a program using DLGSUP has to include DLGSUP in its own logging
cat array (typically not needed for DL* categories), which means for osmo-nitb
that DLGSUP (11) replaces DMGCP (also 11), and DMGCP becomes unusable.

Fix this: make DLGSUP -11 and include in internal_cat.

In gsup_test.c, no longer add DLGSUP to the logging categories array.

External follow-ups are otherwise needed only in osmo-hlr.git and some pending
patches for openbsc (Id3938267fa062e1a997d3704cd678874306f86ee).

Change-Id: Id974c7be158e4d60421a98110f5c807aefd31119
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index ad775b9..cc3919b 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -92,7 +92,7 @@
 #define DLCTRL		-8	/*!< Control Interface */
 #define DLGTP		-9	/*!< GTP (GPRS Tunneling Protocol */
 #define DLSTATS		-10	/*!< Statistics */
-#define DLGSUP		11	/*!< Generic Subscriber Update Protocol */
+#define DLGSUP		-11	/*!< Generic Subscriber Update Protocol */
 #define OSMO_NUM_DLIB	11	/*!< Number of logging sub-systems in libraries */
 
 /*! Configuration of singgle log category / sub-system */