pgsql: Add generate_subscripts, a series-generation function which

Log Message:
-----------
Add generate_subscripts, a series-generation function which generates an
array's subscripts.

Pavel Stehule, some editorialization by me.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        array.sgml (r1.65 -> r1.66)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/array.sgml?r1=1.65&r2=1.66)
        func.sgml (r1.433 -> r1.434)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.433&r2=1.434)
    pgsql/src/backend/utils/adt:
        arrayfuncs.c (r1.143 -> r1.144)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/arrayfuncs.c?r1=1.143&r2=1.144)
    pgsql/src/include/catalog:
        pg_proc.h (r1.492 -> r1.493)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.492&r2=1.493)
    pgsql/src/include/utils:
        array.h (r1.66 -> r1.67)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/array.h?r1=1.66&r2=1.67)
    pgsql/src/test/regress/expected:
        arrays.out (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/arrays.out?r1=1.35&r2=1.36)
    pgsql/src/test/regress/sql:
        arrays.sql (r1.27 -> r1.28)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/arrays.sql?r1=1.27&r2=1.28)