pgsql: * Add support NULL to GiST.

Log Message:
-----------
* Add support NULL to GiST.
* some refactoring and simplify code int gistutil.c and gist.c
* now in some cases it can be called used-defined
  picksplit method for non-first column in index, but here
    is a place to do more.
* small fix of docs related to support NULL.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        indexam.sgml (r2.11 -> r2.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indexam.sgml.diff?r1=2.11&r2=2.12)
        indices.sgml (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/indices.sgml.diff?r1=1.56&r2=1.57)
    pgsql/src/backend/access/gist:
        gist.c (r1.136 -> r1.137)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.136&r2=1.137)
        gistget.c (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.56&r2=1.57)
        gistutil.c (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistutil.c.diff?r1=1.13&r2=1.14)
    pgsql/src/include/access:
        gist_private.h (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.15&r2=1.16)
    pgsql/src/include/catalog:
        catversion.h (r1.333 -> r1.334)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.333&r2=1.334)
        pg_am.h (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_am.h.diff?r1=1.42&r2=1.43)