cleanup: Refactor select_limit in select lex

Replace
  * select_lex::offset_limit
  * select_lex::select_limit
  * select_lex::explicit_limit
with select_lex::Lex_select_limit

The Lex_select_limit already existed with the same elements and was used in
by the yacc parser.

This commit is in preparation for FETCH FIRST implementation, as it
simplifies a lot of the code.

Additionally, the parser is simplified by making use of the stack to
return Lex_select_limit objects.

Cleanup of init_query() too. Removes explicit_limit= 0 as it's done a bit later
in init_select() with limit_params.empty()

sql/structs.h

@@ -824,13 +824,15 @@ public:
 class Lex_select_limit
 {
 public:
+  /* explicit LIMIT clause was used */
   bool explicit_limit;
   Item *select_limit, *offset_limit;
 
-  void empty()
+  void clear()
   {
-    explicit_limit= FALSE;
-    select_limit= offset_limit= NULL;
+    explicit_limit= FALSE;    // No explicit limit given by user
+    select_limit= NULL;       // denotes the default limit = HA_POS_ERROR
+    offset_limit= NULL;       // denotes the default offset = 0
   }
 };