Bug 17234: Need to separate KEY and FOREIGN KEY checks

Bug 17234: Need to separate KEY and FOREIGN KEY checks

In the previous patch we use the constraint_exists subroutine to verify
if an index or a foreign key exists.
But the `SHOW INDEX` query does not return foreign keys (as its name
suggests!).
We need another subroutine foreign_key_exists to check the FK existence.

I have found that because t/db_dependent/TestBuilder.t fails on
oai_sets_biblios, because oai_sets_biblios_ibfk_1 has not been removed.

Test plan:
0/ Do not apply this patch
1/ Use a 3.20 DB
2/ update the DB
3/ SHOW CREATE TABLE oai_sets_biblios
will display oai_sets_biblios_ibfk_1

Apply the patch and repeat 1, 2, 3
=> Will not display oai_sets_biblios_ibfk_1
It has been removed as expected.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>