Allow internal sorts to be stored in memory rather than in files.

src/backend/executor/nodeMergejoin.c

src/backend/executor/nodeSort.c

src/backend/tcop/postgres.c

src/backend/utils/sort/lselect.c

src/backend/utils/sort/psort.c

src/include/nodes/execnodes.h

src/include/nodes/plannodes.h

src/include/utils/lselect.h

src/include/utils/psort.h

src/man/postgres.1

src/man/postmaster.1