WL#3527: Extend IGNORE INDEX so places where index is ignored

can be specified
Currently MySQL allows one to specify what indexes to ignore during
join optimization. The scope of the current USE/FORCE/IGNORE INDEX 
statement is only the FROM clause, while all other clauses are not 
affected.

However, in certain cases, the optimizer
may incorrectly choose an index for sorting and/or grouping, and
produce an inefficient query plan.

This task provides the means to specify what indexes are
ignored/used for what operation in a more fine-grained manner, thus
making it possible to manually force a better plan. We do this
by extending the current IGNORE/USE/FORCE INDEX syntax to:

IGNORE/USE/FORCE INDEX [FOR {JOIN | ORDER | GROUP BY}]

so that:
- if no FOR is specified, the index hint will apply everywhere.
- if MySQL is started with the compatibility option --old_mode then
  an index hint without a FOR clause works as in 5.0 (i.e, the 
  index will only be ignored for JOINs, but can still be used to
  compute ORDER BY).

See the WL#3527 for further details.

sql/sql_class.h

@@ -240,6 +240,11 @@ struct system_variables
 
   my_bool low_priority_updates;
   my_bool new_mode;
+  /* 
+    compatibility option:
+      - index usage hints (USE INDEX without a FOR clause) behave as in 5.0 
+  */
+  my_bool old_mode;
   my_bool query_cache_wlock_invalidate;
   my_bool engine_condition_pushdown;
   my_bool innodb_table_locks;
@@ -277,6 +282,7 @@ struct system_variables
   DATE_TIME_FORMAT *datetime_format;
   DATE_TIME_FORMAT *time_format;
   my_bool sysdate_is_now;
+
 };