More FK constraint stuff

This is mostly the same as an earlier patch I
didn't hear anything about, but which would
have broken with the function manager changes
anyway.

Well, this patch checks that a unique constraint
of some form (unique or pk) is on the referenced
columns of an FK constraint and that the columns
in the referencing table exist at creation time.
The former is to move closer to SQL compatibility
and the latter is in answer to a bug report.
I also added a basic check of this functionality
to the alter table and foreign key regression
tests.

Stephan Szabo
sszabo(at)bigpanda(dot)com