Start the merging-rebase to cygwin-3.4.6

The long-delayed upgrade of Git for Windows' fork of the MSYS2 runtime
to v3.4.* is finally here. And it comes with quite a few changes. The
following two patches made it into the MSYS2 runtime proper:

- 589fac6 (Fix incorrect path conversion (trailing slash),
  2018-03-14) as msys2/msys2-runtime#103 and
  msys2/msys2-runtime#117

- 3e9c003 (Pass environment variables with empty values, 2015-02-18)
  as msys2/msys2-runtime#101

The following two patches made it even further upstream, into Cygwin
proper:

- fa95548 (Implicitly support the /dev/fd symlink and friends,
  2022-02-20), in Cygwin as 4ec0889 (Cygwin: Implicitly support the
  /dev/fd symlink and friends, 2022-02-21)

- 4704d29 (dumper: avoid linker problem when `libbfd` depends on
  `libsframe`, 2023-01-31) was upstreamed as 9acb704afe (dumper: avoid
  linker problem when `libbfd` depends on `libsframe`, 2023-01-31)

Finally, we backported the commit 4d62197 (Cygwin: console: Fix
hangup of less on quit after the window is resized., 2022-12-22), so
naturally it is no longer necessary to carry it in Git for Windows'
fork.

This commit starts the rebase of 353f6b9 to 8dd31e7d1a

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