Alter Table phase 1 -- Please apply to 7.5

Completes:
ALTER TABLE ADD COLUMN does not honour DEFAULT and non-CHECK
CONSTRAINT

ALTER TABLE ADD COLUMN column DEFAULT should fill existing rows
with DEFAULT value

ALTER TABLE ADD COLUMN column SERIAL doesn't create sequence
because of the item above

Previously described reorganization of all ALTER TABLE commands.

Most of the way through column type change. I need to supply a followup
patch which deals with logical attribute numbers.

ALTER TABLE table ALTER [COLUMN] column TYPE type USING expression;

Syntax documentation updates only. Content to come later.