CHECK_FIELD_IGNORE was treated as CHECK_FIELD_ERROR_FOR_NULL;
UPDATE...SET...NULL on NOT NULL fields behaved differently after
a trigger.

Now distinguishes between IGNORE and ERROR_FOR_NULL and save/restores
check-field options.

mysql-test/r/trigger.result:
  Show that UPDATE...SET...NULL on NOT NULL columns doesn't behave differently
  when run after a trigger.
mysql-test/t/trigger.test:
  Show that UPDATE...SET...NULL on NOT NULL columns doesn't behave differently
  when run after a trigger.
sql/field_conv.cc:
  CHECK_FIELD_IGNORE was treated as CHECK_FIELD_ERROR_FOR_NULL.
  Distinguish between the two.
sql/sp_head.cc:
  raise error as needed
sql/sql_class.cc:
  Save and restore check-fields options.
sql/sql_class.h:
  Make room so we can save check-fields options.
sql/sql_insert.cc:
  raise error as needed