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_p.h b/src/log_p.h
index 5cb6aa0..d142131 100644
--- a/src/log_p.h
+++ b/src/log_p.h
@@ -42,6 +42,7 @@
 #define WRN(format,args...) if(verbose_level>=NC_VERB_WARNING){prv_printf(NC_VERB_WARNING,format,##args);}
 #define VRB(format,args...) if(verbose_level>=NC_VERB_VERBOSE){prv_printf(NC_VERB_VERBOSE,format,##args);}
 #define DBG(format,args...) if(verbose_level>=NC_VERB_DEBUG){prv_printf(NC_VERB_DEBUG,format,##args);}
+#define DBL(format,args...) if(verbose_level>=NC_VERB_DEBUG_LOWLVL){prv_printf(NC_VERB_DEBUG_LOWLVL,format,##args);}
 
 #define ERRMEM ERR("%s: memory reallocation failed (%s:%d).", __func__, __FILE__, __LINE__)
 #define ERRARG(arg) ERR("%s: invalid argument (%s).", __func__, arg)