libyang REFACTOR avoid constant literals

Improve readability of the code.

Includes also unification of "no break in the case" comment to "fall
through".
diff --git a/src/diff.c b/src/diff.c
index 347ebf8..f0dcbda 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1300,7 +1300,7 @@
     case LYD_DIFF_OP_REPLACE:
         /* similar to none operation but also remove the redundant attribute */
         lyd_diff_del_meta(diff_match, "orig-value");
-    /* fallthrough */
+    /* fall through */
     case LYD_DIFF_OP_NONE:
         /* it was not modified, but should be deleted -> set DELETE operation */
         LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_DELETE));