Support window functions a la SQL:2008.

contrib/tsearch2/tsearch2.c

doc/src/sgml/advanced.sgml

doc/src/sgml/errcodes.sgml

doc/src/sgml/func.sgml

doc/src/sgml/queries.sgml

doc/src/sgml/query.sgml

doc/src/sgml/ref/select.sgml

doc/src/sgml/ref/select_into.sgml

doc/src/sgml/syntax.sgml

doc/src/sgml/xaggr.sgml

src/backend/catalog/dependency.c

src/backend/catalog/heap.c

src/backend/catalog/pg_proc.c

src/backend/commands/explain.c

src/backend/commands/functioncmds.c

src/backend/commands/prepare.c

src/backend/commands/tablecmds.c

src/backend/commands/typecmds.c

src/backend/executor/Makefile

src/backend/executor/execAmi.c