Update timezone code from tzcode 2020a

This patch updates files coming from tzcode to tzcode 2020a.
This is mostly for better support for Internet RFC 8536, by adding
support to zic for the Expires line (new to tzcode 2020a), the -b
option (new to 2019b) and the -r option (new to 2019a).
One trivial change to other glibc was needed.
* time/tzfile.c (__tzfile_read): Adjust to tzcode private.h renaming.
* timezone/private.h, timezone/tzfile.h, timezone/version:
* timezone/zdump.c, timezone/zic.c: Update from tzcode 2020a.