Enhancement: Support pruning even when the disk is full

When running out of disk space it was no longer possible to add or remove
data from a repository. To help with recovering from such a deadlock, the
prune command now supports an `--unsafe-recover-no-free-space` option to
recover from these situations. Make sure to read the documentation first!

https://github.com/restic/restic/issues/1153
https://github.com/restic/restic/pull/3481
