minor tz tuneups and portability fix

[Robert Elz]

    Date:        Sun, 14 Jan 1996 22:37:20 -0800
    From:        Paul Eggert <eggert at twinsun.com>
    Message-ID:  <199601150637.WAA15042 at der.twinsun.com>

    While accumulating these I noticed a portability bug in localtime.c;
    the expression `(t < 0) ? 0 : ((time_t) 1 << bits)' yields undefined
    results if time_t is unsigned, since `bits' is the number of bits in
    a time_t in that case.  The patch below fixes this bug as part of the tuneups.

It is a "portability bug" in the sense that it is simply wrong
if time_t is unsigned (but OK for signed time_t).  I am not
sure how this happened - I actually had unsigned time_t for
a while (and it was in some BSD beta release ages ago, before
people west of Greenwich complained about what that meant to
((time_t)0) when converted to local time...

That is, once upon a time, this code was actually tested, and
worked, with unsigned time_t, somehow it seems to have suffered
some bit rot...   Unfortunately I don't have any very ancient
versions to look see what happened there.   (bits - 1) is
clearly the right thing (1 << bits) is going to be either 0 or
1 on most architectures (but might be anything).

kre