drop-upload: optionally unlink locally deleted files from upload directory

[davidsarah]

The prototype implementation of drop-upload in #1429 adds new or changed files in the local directory to the upload directory, but it does not unlink files in the upload directory when they are deleted locally.

Note that:

• some users might not want files to be immediately unlinked from the upload directory, in case the local deletion was accidental.
• the space for unlinked files would not be reclaimed unless and until garbage collection is performed. (This applies already to old versions of changed files.)