Anonymous usage statistics

Get anonymous usage statistics from users.

Obviously, reprozip needs to ask the user to opt-in before reporting anything back to us.

What to report:
* IDs
 * [X] assign anonymous ID to reprozip user
 * [x] assign ID to generated pack
* size of experiment
 * [X] nb packages
 * [X] nb of files in packages
 * [ ] total size of files from packages
 * [ ] total size of packages
 * ~~which packages~~
 * [X] nb of other files
 * [ ] total size of other files
 * [X] execution time
 * [ ] nb of processes?
 * [ ] nb of processes running at any time? (slow?)
* [x] argv0(?)
* [X] unpackers used
* [ ] environment (packing & unpacking)
* [ ] specific actions
 * [x] replace input
 * [x] extract output
  * [ ] size of these?

Made [remram44/usagestats](/remram44/usagestats) for this.