Some #includes were missing for the latter (but only noticeable on some
systems - e.g., build breaks on Cygwin).
So let's replace with something that works equally everywhere.
Fixes: #2060
Until now, mu would _not_ follow symlinks; with these changes, we do.
There were some complications with that ~10 years ago, but I forgot the
details. So let's re-enable. At least one thing is in place now: moving
between file systems.
Fixes#1489Fixes#1628 (technically, this came with slightly earlier commit)