pgsql: Allow I/O conversion casts to be applied to or from any type that

Log Message:
-----------
Allow I/O conversion casts to be applied to or from any type that is a member
of the STRING type category, thereby opening up the mechanism for user-defined
types.  This is mainly for the benefit of citext, though; there aren't likely
to be a lot of types that are all general-purpose character strings.
Per discussion with David Wheeler.

Modified Files:
--------------
    pgsql/contrib/citext/expected:
        citext.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/citext/expected/citext.out?r1=1.1&r2=1.2)
    pgsql/contrib/citext/sql:
        citext.sql (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/citext/sql/citext.sql?r1=1.1&r2=1.2)
    pgsql/doc/src/sgml/ref:
        create_cast.sgml (r1.28 -> r1.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_cast.sgml?r1=1.28&r2=1.29)
    pgsql/src/backend/parser:
        parse_coerce.c (r2.162 -> r2.163)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_coerce.c?r1=2.162&r2=2.163)