Start the merging-rebase to v2.21.0

We use this opportunity to move the following topics into the
`ready-for-upstream` branch thicket: msvc, visual-studio,
no-perl-makemaker, program-data-config and ansi-unicode.

This commit starts the rebase of a4b8189 to 2bb6486

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho