Fourth part of fix for bug#25722927 "NEWDD FK: ALTER TABLE CHANGE COLUMN TYPE SHOULD CHECK FK CONSTRAINT" [noclose]

After we have started to store information about foreign keys in the
Transactional Data Dictionary it became possible to move checks of
foreign key definition validity from the storage engine to SQL-layer
and thus reduce code duplication and simplify SE implementation.

Moved check which prohibits foreign keys with SET NULL referential
action and non-nullable referencing columns to SQL-layer. CREATE TABLE
now emits better, more specific errors in such cases as result.

Removed InnoDB code which did this and other checks which became
redundant after this and earlier patches for this bug.

Added new and adjusted existing test coverage accordingly.