schema parsers CHANGE checking for allowed place for mandatory statement

There are 2 situations handled (better) now:
- mandatory nodes in augment are allowed only when augment is made
  conditional by when statement
- mandatory nodes are not allowed under the default choice's case

Fixes #99
diff --git a/src/parser.h b/src/parser.h
index 6251fb3..af336d1 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -96,7 +96,8 @@
 
 int lyp_check_identifier(const char *id, enum LY_IDENT type, struct lys_module *module, struct lys_node *parent);
 int lyp_check_date(const char *date);
-int lyp_check_mandatory(struct lys_node *node);
+int lyp_check_mandatory_augment(struct lys_node_augment *node);
+int lyp_check_mandatory_choice(struct lys_node *node);
 
 int lyp_check_include(struct lys_module *module, struct lys_submodule *submodule, const char *value,
                       struct lys_include *inc, struct unres_schema *unres);