pgsql: Add btree_gist support for searching on "not equals".

Log Message:
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Add btree_gist support for searching on "not equals".

Jeff Davis, with slight editorialization by me.

Modified Files:
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    pgsql/contrib/btree_gist:
        btree_gist.h (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/btree_gist/btree_gist.h?r1=1.9&r2=1.10)
        btree_gist.sql.in (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/btree_gist/btree_gist.sql.in?r1=1.21&r2=1.22)
        btree_utils_num.c (r1.12 -> r1.13)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/btree_gist/btree_utils_num.c?r1=1.12&r2=1.13)
        btree_utils_var.c (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/btree_gist/btree_utils_var.c?r1=1.23&r2=1.24)
    pgsql/doc/src/sgml:
        btree-gist.sgml (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/btree-gist.sgml?r1=1.5&r2=1.6)