This worklog populates the new dictionary tables for foreign keys
(mysql.foreign_keys and mysql.foreign_key_column_usage) whenever
a foreign key is created or dropped. Note that foreign key information
will not be stored for storage engines that do not support foreign
keys (e.g. MyISAM) - however syntax checking will still be done.

This worklog is limited in scope - the new dictionary tables
for foreign keys are not used for I_S (see WL#6599) or by
InnoDB (see WL#7595) yet. This worklog also does not implement
metadata locking for foreign keys (see WL#6049).

This worklog does introduce some user visible changes of behavior:
- ER_FK_DUP_NAME is now reported more consistently in cases where
  duplicate foreign key names exist. Foreign key names have to be
  unique for a given schema.
- Foreign key names are now limited to 64 characters similar to
  other identifiers.
- If no foreign key name is given, a name is generated. For now
  we use the same name as generated by InnoDB. The format is
  (table_name)_ibfk_(counter).
- More validity checking is done for foreign key definitions
  created for tables with storage engines that do not support
  foreign keys (even if the fks are not stored).

Note that as metadata locking for foreign keys has not been
implemented yet, any relevant changes to the parent table
are not reflected in the foreign key definition. This will be
fixed later after WL#6049 has been implemented.