pgsql: Add items from Tom: < o Add a GUC variable to allow output of

Log Message:
-----------
Add items from Tom:

<     o Add a GUC variable to allow output of interval values in ISO8601
<       format
212a211,223
>     o Add a GUC variable to allow output of interval values in ISO8601
>       format
>     o Improve timestamptz subtraction to be DST-aware
>
>       Currently, subtracting one date from another that crosses a
>       daylight savings time adjustment can return '1 day 1 hour', but
>       adding that back to the first date returns a time one hour in
>       the future.  This is caused by the adjustment of '25 hours' to
>       '1 day 1 hour', and '1 day' is the same time the next day, even
>       if daylight savings adjustments are involved.
>
>     o Fix interval display to support values exceeding 2^31 hours
>     o Add overflow checking to timestamp and interval arithmetic

Modified Files:
--------------
    pgsql/doc:
        TODO (r1.1693 -> r1.1694)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1693&r2=1.1694)
    pgsql/doc/src/FAQ:
        TODO.html (r1.200 -> r1.201)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.200&r2=1.201)