Note that the `fd` is only valid while the file is still open. So we need to
move the setting calls to before we close the file. However! We cannot do so
with the `utime()` call (even though `futimens()` exists) because the follow-
ing `close()` call to the `fd` will reset the atime of the file. So it seems
the `utime()` call has to happen after the file is closed.
Somewhat surprisingly, calling `fchmod()` is non-trivially faster than calling
`chmod()`, and so on.
This commit introduces alternate variants to some common file util functions
that take an optional fd. If present, they call the `f`-variant of the
underlying function. Otherwise, they fall back to the regular filename-taking
version of the function.
Basic tests for (de)compressing in the following modes:
* file to file
* file to stdout
* stdin to file
* stdin to stdout
These are basic tests, and aren't testing more advanced scenarios, but
it adds the groundwork for more complex tests as needed.
Fixes#3010.
