Version 2021.3

Upstream version 2021c released 2021-10-01T21:21:49+00:00

Briefly:

Revert most 2021b changes to 'backward'. Fix 'zic -b fat' bug in pre-1970 32-bit
data. Fix two Link line typos. Distribute SECURITY file.

This release is intended as a bugfix release, to fix compatibility problems and
typos reported since 2021b was released.

Changes to Link directives

Revert almost all of 2021b's changes to the 'backward' file, by moving Link
directives back to where they were in 2021a. Although 'zic' doesn't care which
source file contains a Link directive, some downstream uses ran into trouble
with the move. (Problem reported by Stephen Colebourne for Joda-Time.)

Fix typo that linked Atlantic/Jan_Mayen to the wrong location (problem reported
by Chris Walton).

Fix 'backzone' typo that linked America/Virgin to the wrong location (problem
reported by Michael Deckers).

Changes to documentation

Distribute the SECURITY file (problem reported by Andreas Radke).