Feature request: add dereference option for /storage/xfer-internal/cp

[khsrali]

When copying files within internal CSCS systems, usually it's useful to preserve all symlinks to avoid overfilling storage.
This is the default behaviour of the well developed /storage/xfer-internal/cp.

However, in some use cases, such as stashing, it might be useful to allow cp -L by dereferencing symlinks. This way, if the original data is deleted, the stored data remains correct.

Although this feature is not absolutely necessary now, it might be useful to consider for future developments.