When processing the USE/FORCE index hints
the optimizer was not checking if the indexes 
specified are enabled (see ALTER TABLE).
Fixed by:
 Backporting the fix for bug 20604 to 5.0


mysql-test/r/key.result:
  Test for BUG 20604.
  The important part of the test is the explain output that 
  tests what indexes are used.
mysql-test/r/myisam.result:
  Bug #28476: test cases
mysql-test/t/key.test:
  Bug 20604: 
  The minimal test case that reveals the bug. The optimizer for 
  aggregates relies on keys disabled with ALTER TABLE ... DISABLE KEYS
  not being in the set TABLE::keys_in_use_for_query.
  When the execution engine tries to use a disabled index, MyISAM
  returns an error.
mysql-test/t/myisam.test:
  Bug #28476: test cases
sql/sql_base.cc:
  Bug #28476: 
   - Ignore disabled indexes in USE/FORCE index
sql/sql_select.cc:
  Bug 20604 : The intersection operation between table->s->keys_in_use 
  and table->keys_in_use_for_query is no longer necessary.
  We can trust that the latter is a subset of the former.
sql/table.h:
  Bug 20604:
  Added comments to TABLE_SHARE::keys_in_use and
  TABLE::keys_in_use_for_query.