log CHANGE new low-level debug log level

For messages that should be explicitly requested
because they significantly (printed periodically)
clutter the output.

Fixes cesnet/netopeer2#591
diff --git a/src/log.c b/src/log.c
index 614efcc..57ca05c 100644
--- a/src/log.c
+++ b/src/log.c
@@ -45,7 +45,8 @@
     {NC_VERB_ERROR, "[ERR]"},
     {NC_VERB_WARNING, "[WRN]"},
     {NC_VERB_VERBOSE, "[INF]"},
-    {NC_VERB_DEBUG, "[DBG]"}
+    {NC_VERB_DEBUG, "[DBG]"},
+    {NC_VERB_DEBUG_LOWLVL, "[DBL]"}
 };
 
 #ifdef NC_ENABLED_SSH