Add ALTER TABLE <tablename> CLUSTER ON <indexname>

doc/src/sgml/ref/alter_table.sgml

src/backend/commands/tablecmds.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/include/commands/tablecmds.h

src/include/nodes/parsenodes.h

src/test/regress/expected/cluster.out

src/test/regress/sql/cluster.sql