isup: Start parsing the ISUP messages

Introduce a ISUP debug category, parse the reset circuit
message, add a test case for this easy parsing.
diff --git a/src/debug.c b/src/debug.c
index 4382a47..93f4160 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -45,6 +45,11 @@
 		.description = "Media Gateway Control Protocol",
 		.enabled = 1, .loglevel = LOGL_NOTICE,
 	},
+	[DISUP] = {
+		.name = "DISUP",
+		.description = "ISUP",
+		.enabled = 1, .loglevel = LOGL_NOTICE,
+	},
 };
 
 static int filter_fn(const struct log_context *ctx,