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/hash_table.h b/src/hash_table.h
index 8e135b0..2e7a511 100644
--- a/src/hash_table.h
+++ b/src/hash_table.h
@@ -49,6 +49,9 @@
  */
 typedef ly_bool (*values_equal_cb)(void *val1_p, void *val2_p, ly_bool mod, void *cb_data);
 
+/** reference value for 100% */
+#define LYHT_HUNDRED_PERCENTAGE 100
+
 /** when the table is at least this much percent full, it is enlarged (double the size) */
 #define LYHT_ENLARGE_PERCENTAGE 75