DROP IF EXISTS for columns and constraints. Andres Freund.

doc/src/sgml/ref/alter_table.sgml

src/backend/commands/tablecmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/include/nodes/parsenodes.h

src/test/regress/expected/alter_table.out

src/test/regress/sql/alter_table.sql