Start the merging-rebase to cygwin-3_1_6-release

There is not actually a new Cygwin runtime version, it is still v3.1.6,
but we want to pick up a couple of changes from the MSYS2 runtime
project (e.g. the nice GitHub PR workflow).

To make that more obvious, we now merge the `msys2-3_1_6-release` (which
is a branch based on `cygwin-3_1_6-release`) first thing.

This commit starts the rebase of 13ae584 to d11146c6e

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>