Rebase to tzdata-2021c - supersedes tzdata-2021b
- Revert most of 2021b changes to 'backward'.
- Fix 'zic -b fat' bug in pre-1970 32-bit data.
- Fix two Link line typos.
- Distribute SECURITY file.
- Includes changes in tzdata-2021b:
- Jordan now starts DST on February's last Thursday.
- Samoa no longer observes DST.
- Merge more location-based Zones whose timestamps agree since 1970.
- Move some backward-compatibility links to 'backward'.
- Rename Pacific/Enderbury to Pacific/Kanton.
- Correct many pre-1993 transitions in Malawi, Portugal, etc.
- zic now creates each output file or link atomically.
- zic -L no longer omits the POSIX TZ string in its output.
- zic fixes for truncation and leap second table expiration.
- zic now follows POSIX for TZ strings using all-year DST.
- Fix some localtime crashes and bugs in obscure cases.
- zdump -v now outputs more-useful boundary cases.
- tzfile.5 better matches a draft successor to RFC 8536.
