* Changed debug levels in clicon_debug() to be based on maskable flags

* Added flag names: `CLIXON_DBG_*`
  * Added maskable flags that can be combined when debugging:
    * `DEFAULT` = 1: Basic debug message, espcially initialization
    * `MSG` = 2: Input and output packets, read datastore
    * `DETAIL` = 4: Details: message dump in hex, xpath parse trees, etc
    * `EXTRA` = 8: Extra detailed logs
* Test: some errors in yang-lib where content-id was in wrong place

lib/src/clixon_xml_io.c

@@ -541,7 +541,7 @@ _xml_parse(const char *str,
     int             failed = 0; /* yang assignment */
     int             i;
 
-    clicon_debug(2, "%s", __FUNCTION__);
+    clicon_debug(CLIXON_DBG_DETAIL, "%s", __FUNCTION__);
     if (strlen(str) == 0){
         return 1; /* OK */
     }