pgsql: Repair two TIME WITH TIME ZONE bugs found by Dennis Vshivkov.

Log Message:
-----------
Repair two TIME WITH TIME ZONE bugs found by Dennis Vshivkov.  Comparison
of timetz values misbehaved in --enable-integer-datetime cases, and
EXTRACT(EPOCH) subtracted the zone instead of adding it in all cases.
Backpatch to all supported releases (except --enable-integer-datetime code
does not exist in 7.2).

Tags:
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REL7_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        date.c (r1.73.2.7 -> r1.73.2.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c.diff?r1=1.73.2.7&r2=1.73.2.8)