- BUG#15166: Wrong update permissions required to execute triggers
  - BUG#15196: Wrong select permission required to execute triggers

The idea of the fix is to check necessary privileges
in Item_trigger_field::fix_fields(), instead of having "special variables"
technique. To achieve this, we should pass to an Item_trigger_field instance
a flag, which will indicate the usage/access type of this trigger variable.


mysql-test/r/trigger-grant.result:
  Update the result file.
mysql-test/t/trigger-grant.test:
  Add test cases for BUG#15166 and BUG#15196
sql/item.cc:
  Item_trigger_field: check appropriate (SELECT/UPDATE) privilege in fix_fields().
sql/item.h:
  Add a flag to specify access type for trigger field.
sql/sql_trigger.cc:
  "Special variable" technique of checking privileges for NEW/OLD variables
  was replaced by checking table- and column-level privileges in
  Item_trigger_field::fix_fields().
sql/sql_trigger.h:
  "Special variable" technique of checking privileges for NEW/OLD variables
  was replaced by checking table- and column-level privileges in
  Item_trigger_field::fix_fields().
sql/sql_yacc.yy:
  Specify access type for trigger fields.