data tree CHANGE modify lyd_leaf_type()

change the function to return not only the data type enumeration but
the complete data type definition. This allows caller to get detailed
information about the type restrictions.

Relates to CESNET/Netopeer2#47
diff --git a/src/tree_schema.c b/src/tree_schema.c
index e5f00b7..c905a5b 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -3703,3 +3703,4 @@
 
     return result;
 }
+