pgsql: Make 'col IS NULL' clauses be indexable conditions.

Log Message:
-----------
Make 'col IS NULL' clauses be indexable conditions.

Teodor Sigaev, with some kibitzing from Tom Lane.

Modified Files:
--------------
pgsql/doc/src/sgml:
catalogs.sgml (r2.149 -> r2.150)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.149&r2=2.150)
indexam.sgml (r2.22 -> r2.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indexam.sgml.diff?r1=2.22&r2=2.23)
indices.sgml (r1.70 -> r1.71)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indices.sgml.diff?r1=1.70&r2=1.71)
pgsql/doc/src/sgml/ref:
create_index.sgml (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_index.sgml.diff?r1=1.61&r2=1.62)
pgsql/src/backend/access/common:
scankey.c (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/scankey.c.diff?r1=1.29&r2=1.30)
pgsql/src/backend/access/gist:
gistget.c (r1.64 -> r1.65)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.64&r2=1.65)
pgsql/src/backend/access/nbtree:
nbtsearch.c (r1.111 -> r1.112)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.111&r2=1.112)
nbtutils.c (r1.83 -> r1.84)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.83&r2=1.84)
pgsql/src/backend/executor:
nodeIndexscan.c (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeIndexscan.c.diff?r1=1.120&r2=1.121)
pgsql/src/backend/optimizer/path:
indxpath.c (r1.218 -> r1.219)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.218&r2=1.219)
pgsql/src/backend/optimizer/plan:
createplan.c (r1.227 -> r1.228)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/createplan.c.diff?r1=1.227&r2=1.228)
pgsql/src/backend/optimizer/util:
plancat.c (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/plancat.c.diff?r1=1.132&r2=1.133)
pgsql/src/backend/utils/adt:
selfuncs.c (r1.231 -> r1.232)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.231&r2=1.232)
pgsql/src/include/access:
skey.h (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/skey.h.diff?r1=1.34&r2=1.35)
pgsql/src/include/catalog:
catversion.h (r1.400 -> r1.401)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.400&r2=1.401)
pg_am.h (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_am.h.diff?r1=1.50&r2=1.51)
pgsql/src/include/nodes:
relation.h (r1.139 -> r1.140)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h.diff?r1=1.139&r2=1.140)
pgsql/src/test/regress/expected:
create_index.out (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/create_index.out.diff?r1=1.23&r2=1.24)
pgsql/src/test/regress/sql:
create_index.sql (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/create_index.sql.diff?r1=1.22&r2=1.23)