srfi-19 default tz offset limit

[Kevin Ryde]

I added these words to srfi-19 conversions like time-utc->date.  I
thought about actually implementing the advice given, but decided it
was too much work for what would be imperfect anyway.


     For conversions to dates, TZ-OFFSET is seconds east of Greenwich.
     The default is the local timezone, at the given time, as provided
     by the system, using `localtime' (*note Time::).

     On 32-bit systems, `localtime' is limited to a 32-bit `time_t', so
     a default TZ-OFFSET is only available for times between Dec 1901
     and Jan 2038.  For prior dates an application might like to use
     the value in 1902, though some locations have zone changes prior
     to that.  For future dates an application might like to assume
     today's rules extend indefinitely.  But for correct daylight
     savings transitions it will be necessary to take an offset for the
     same day and time but a year in range and which has the same
     starting weekday and same leap/non-leap (to support rules like
     last Sunday in October).