Paramiko seems to have had a slight change in behavior that broke
devlib: to save a remote command execution, we attempt to pull any path
as file first, and then as a folder if the former failed.

This is now broken as paramiko will create an empty destination file
when trying to pull as a file. When we attempt again to pull as folder,
the destination exists already (empty file) and we raise an exception.

To fix that, make sure we cleanup any attempt after pulling as a file
before trying again.

When pulling a file from the target, copy all paths as files and follow
symlinks if necessary. That fixes issues related to chmod not working on
symlinks and generally allows getting any path.

If we want to one day preserve symlinks in some capacities, we can
always add an option to Target.pull() to do so.

Ensure we use the busybox command in operations involved in file
transfers.

* Combine cp and chmod for pull
* Make both push and pull use concurrent async code

_prepare_xfer() deals with all the paths resulting from glob expansion,
so it can benefit from async capabilities in order to process multiple
files concurrently.

Convert the internals to async/await to enable useful map_concurrently()