CLUSTER VERBOSE and corresponding clusterdb --verbose option

doc/src/sgml/ref/cluster.sgml

doc/src/sgml/ref/clusterdb.sgml

src/backend/commands/cluster.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/bin/scripts/clusterdb.c

src/include/nodes/parsenodes.h