Completed pg_depend support

Tracks most inter system table dependencies.

With a small amount of work, allows dropping sequences with the table in
the event of a serial type. Restricting (cascading) through foreign
keys, and functional RESTRICT / CASCADE behaviour to most items.

Doesn't allow DROP USER CASCADE, nor does it track comments. A few
tables like opclass, and am/amop are not tracked either simply because I
really don't know what they do and how the user can affect them.

Rather large regression tests will be started with RESTRICT / CASCADE
behaviours after this portion is committed. Tired of getting a large
number of conflicts every couple days with Toms namespace work.

New tricks? Check out heap_drop_with_catalog. You'll notice the
relation type, indicies, view rules are handled by dependDelete().

Also, try to drop type int4 ;) Restricts by default (fresh initdb).

template1=# drop type int4;
ERROR: Drop Restricted as Table pg_inherits Depends on Type int4

TODO.depend is the list of actions I needed to do (or did). More or
gives a list of affected areas.

Dependencies still to do are in precompiled copies of functions,
defaults, and rule contents. I have no clues how to do those items.