Command line options: unlink on copyup

[kcgthb]

Provide a command-line option to enable/disable unlink on copyup.

When migrating data from one filesystem to another, one objective may be to empty up the lower layer. It could be done via purge policies on that layer, but it could also be accelerated by removing files that are copied-up from the lower layer.

Some sites may prefer to keep the lower version of copied-up files intact, as an extra copy, or to manage lower filesystem cleanup a different way.

[corbinjurgens]

I think this would be good