yanglint FEATURE initial version of yanglint(1)

Source codes are mostly taken from libyang 1.0 and slightly modified for
libyang 2.0. A lot of functionality is still missing since it is not yet
provided by libyang 2.0
diff --git a/src/tree_schema_internal.h b/src/tree_schema_internal.h
index 8b7d66b..e15f7cd 100644
--- a/src/tree_schema_internal.h
+++ b/src/tree_schema_internal.h
@@ -129,7 +129,9 @@
  * @param[in] ctx libyang context.
  * @param[in] name Name of the module to load.
  * @param[in] revison Optional revision of the module to load. If NULL, the newest revision is loaded.
- * @param[in] implement Flag if the loaded module is supposed to be marked as implemented.
+ * @param[in] implement Flag if the loaded module is supposed to be marked as implemented. If revision is NULL and implement flag set,
+ * the implemented module in the context is returned despite it might not be of the latest revision, because in this case the module
+ * of the latest revision can not be made implemented.
  * @param[in] require_parsed Flag to require parsed module structure in case the module is already in the context,
  * but only the compiled structure is available.
  * @param[out] mod Parsed module structure.