foreign key constraint, find a truly equivalent index for it.
If none is available, refuse to drop the index.  MySQL can drop
an index when creating a "stronger" index.

This was reported as Mantis issue #70 and MySQL Bug #38786.

innodb-index.test: Add a test case.

dict_foreign_find_equiv_index(): New function, to replace the
incorrectly written function dict_table_find_equivalent_index().

dict_table_replace_index_in_foreign_list(): Simplify the implementation.