INSERT triggers".

In cases when REPLACE was internally executed via update and table had
on update (on delete) triggers defined we exposed the fact that such
optimization used by callng on update (not calling on delete) triggers.
Such behavior contradicts our documentation which describes REPLACE as
INSERT with optional DELETE.

This fix just disables this optimization for tables with on delete triggers.
The optimization is still applied for tables which have on update but have
no on delete triggers, we just don't invoke on update triggers in this case
and thus don't expose information about optimization to user.

Also added test coverage for values returned by ROW_COUNT() function (and
thus for values returned by mysql_affected_rows()) for various forms of
INSERT.


mysql-test/r/insert.result:
  Added test for values returned by ROW_COUNT() function (and thus for values
  returned by mysql_affected_rows()) for various forms of INSERT. We didn't
  have coverage for this before and since this fix touches related code it is
  better to add it now.
mysql-test/r/trigger.result:
  Adjusted test after fixing bug#13479 "REPLACE activates UPDATE trigger, not
  the DELETE and INSERT triggers".
mysql-test/t/insert.test:
  Added test for values returned by ROW_COUNT() function (and thus for values
  returned by mysql_affected_rows()) for various forms of INSERT. We didn't
  have coverage for this before and since this fix touches related code it is
  better to add it now.
mysql-test/t/trigger.test:
  Adjusted test after fixing bug#13479 "REPLACE activates UPDATE trigger, not
  the DELETE and INSERT triggers".
sql/sql_insert.cc:
  write_record():
    We should not expose that internally we sometimes execute REPLACE
    via UPDATE instead of documented INSERT + DELETE pair. So we should not
    use this optimization for tables with on delete triggers. OTOH it is ok
    to use it for tables which have on update but have no on delete triggers,
    we just should not invoke on update triggers in this case.