BACKGROUND:
With the push of WL#7467, it was observed that SQL_MODE is 
changed to wrong value in triggers during live/dump upgrade
which is not desired behaviour.

ANALYSIS:
SQL_MODE for triggers is stored in .TRG files at trigger 
creation time.
If the user upgraded to the version in which WL#7467 was pushed,
the value of SQL_MODES in .TRG files started mapping to the
new SQL_MODES currently available which was not desired and
might lead to wrong behaviour of triggers after upgrade.

FIX:
As a fix for this issue, the removed SQL_MODES are again
introduced for backward compatibility during upgrade.
However, Setting NO_ZERO_DATE, NO_ZERO_IN_DATE and
ERROR_FOR_DIVISION_BY_ZERO will have no effect and 
user will get a warning if he tries to explicitly set
these modes.  

NOTE TO DOCUMENTATION:
=====================
For stored routines that were created in 5.6 with 
NO_ZERO_* modes, after upgrade to 5.7, these modes will
*not* show up in the output of
SHOW CREATE PROC/FUNC/EVENTS/TRIGGERS, and SELECT * FROM
information_schema.triggers.
Only Select * from mysql.proc and mysql.events
table will show the NO_ZERO_* modes after upgrade for the 
procs/events that were created in 5.6.
In 5.7, since we cannot use NO_ZERO_* modes,
mysql.proc/mysql.events or even SHOW CREATE will not show
any of NO_ZERO_* Modes.