pgsql: Add get_bit/set_bit functions for bit strings, paralleling those

Log Message:
-----------
Add get_bit/set_bit functions for bit strings, paralleling those for bytea,
and implement OVERLAY() for bit strings and bytea.

In passing also convert text OVERLAY() to a true built-in, instead of
relying on a SQL function.

Leonardo F, reviewed by Kevin Grittner

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.497 -> r1.498)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.497&r2=1.498)
    pgsql/src/backend/parser:
        gram.y (r2.704 -> r2.705)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.704&r2=2.705)
    pgsql/src/backend/utils/adt:
        varbit.c (r1.63 -> r1.64)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varbit.c?r1=1.63&r2=1.64)
        varlena.c (r1.173 -> r1.174)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c?r1=1.173&r2=1.174)
    pgsql/src/include/catalog:
        catversion.h (r1.578 -> r1.579)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.578&r2=1.579)
        pg_proc.h (r1.564 -> r1.565)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.564&r2=1.565)
    pgsql/src/include/utils:
        builtins.h (r1.344 -> r1.345)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.344&r2=1.345)
        bytea.h (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/bytea.h?r1=1.2&r2=1.3)
        varbit.h (r1.30 -> r1.31)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/varbit.h?r1=1.30&r2=1.31)
    pgsql/src/test/regress/expected:
        bit.out (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/bit.out?r1=1.4&r2=1.5)
        strings.out (r1.39 -> r1.40)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/strings.out?r1=1.39&r2=1.40)
    pgsql/src/test/regress/sql:
        bit.sql (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/bit.sql?r1=1.2&r2=1.3)
        strings.sql (r1.27 -> r1.28)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/strings.sql?r1=1.27&r2=1.28)