* 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

apps/backend/backend_socket.c

@@ -241,7 +241,7 @@ backend_accept_client(int   fd,
     uid_t                guid;
 #endif
 
-    clicon_debug(2, "%s", __FUNCTION__);
+    clicon_debug(CLIXON_DBG_DETAIL, "%s", __FUNCTION__);
     len = sizeof(from);
     if ((s = accept(fd, &from, &len)) < 0){
         clicon_err(OE_UNIX, errno, "accept");