Extend COPY to support COPY (SELECT ...) TO ...

doc/src/sgml/ref/copy.sgml

doc/src/sgml/ref/psql-ref.sgml

src/backend/commands/copy.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/tcop/dest.c

src/bin/psql/copy.c

src/include/commands/copy.h

src/include/nodes/parsenodes.h

src/include/tcop/dest.h

src/test/regress/expected/copyselect.out

src/test/regress/parallel_schedule

src/test/regress/serial_schedule

src/test/regress/sql/copyselect.sql