Add GUC temp_tablespaces to provide a default location for temporary

doc/src/sgml/config.sgml

src/backend/commands/indexcmds.c

src/backend/commands/tablecmds.c

src/backend/commands/tablespace.c

src/backend/executor/execMain.c

src/backend/storage/file/fd.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/include/commands/tablespace.h

src/include/utils/guc.h