ALTER TABLE SET/DROP NOT NULL, from Christopher Kings-Lynne.

doc/src/sgml/ref/alter_table.sgml

doc/src/sgml/release.sgml

src/backend/commands/command.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/include/commands/command.h

src/include/nodes/parsenodes.h

src/test/regress/expected/alter_table.out

src/test/regress/sql/alter_table.sql