Since tzcode95f (1995), TZif files have had a trailing
TZ string, used for timestamps after the last transition.
This string is specified in Internet RFC 8536 section 3.3.
init_zone_from_iana_info has ignored this string, causing it
to mishandle timestamps past the year 2038.  With zic's new -b
slim flag, init_zone_from_iana_info would even mishandle current
timestamps.  Fix this by parsing the trailing TZ string and adding
its transitions.

Closes #2129