Dependency / Constraint patch

Differences from previous version:
- Fully functional ALTER TABLE / DROP CONSTRAINT
- pg_dump uses ALTER TABLE / ADD FOREIGN KEY
- psql displays foreign keys (\d output)
- Foreign key triggers are autonamed based on the constraint name
- Namespace dependencies were quickly added. Unable to test them very
well (DROP SCHEMA required)

Postgresql TODO items completed (or very close):
# Add ALTER TABLE DROP non-CHECK CONSTRAINT
# Allow psql \d to show foreign keys
* Add pg_depend table for dependency recording; use sysrelid, oid,
depend_sysrelid, depend_oid, name
# Auto-destroy sequence on DROP of table with SERIAL
# Prevent column dropping if column is used by foreign key
# Automatically drop constraints/functions when object is dropped
# Make constraints clearer in dump file
# Make foreign keys easier to identify

The locking of relations may not be as strong as it should be. I was
unable to cause failure -- and can't see what it would be missing but it
has been bothering me.

I've not touched pg_dump for SERIAL stuff. I may do it later.

Basic documentation updates included. I'll scour for examples or notes
which may no longer apply in a couple of weeks.

Attached files:

TODO.depend - A short list of items I completed, notes, any assumptions,
and possible outstanding items

src/backend/catalog/pg_constraint.c
src/backend/catalog/pg_depend.c
src/include/catalog/pg_constraint.h
src/include/catalog/pg_depend.h
src/test/regress/expected/drop.out

Remove:
src/backend/catalog/pg_relcheck.c
src/include/catalog/pg_relcheck.h