Make 'col IS NULL' clauses be indexable conditions.

doc/src/sgml/catalogs.sgml

doc/src/sgml/indexam.sgml

doc/src/sgml/indices.sgml

doc/src/sgml/ref/create_index.sgml

src/backend/access/common/scankey.c

src/backend/access/gist/gistget.c

src/backend/access/nbtree/nbtsearch.c

src/backend/access/nbtree/nbtutils.c

src/backend/executor/nodeIndexscan.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/util/plancat.c

src/backend/utils/adt/selfuncs.c

src/include/access/skey.h

src/include/catalog/catversion.h

src/include/catalog/pg_am.h

src/include/nodes/relation.h

src/test/regress/expected/create_index.out

src/test/regress/sql/create_index.sql