`edit-chroot` fixes

A number of improvements to `edit-chroot`:
 - Make sure all chroots passed as parameters are valid before starting to backup/delete chroots (should give at least some degree of protection against specifying parameters in the wrong order).
 - Delete partial archives if archiving cannot be completed.
 - Unit test for `edit-chroot`.

Other modifications:
 - Make sure chroot names do not contain `/` (this should be handled by a different prefix), as it could cause a range of problems. `.`, `..` and empty string are also invalid names.
 - Make sure chroots can start with `-`: protect `*-chroot` arguments with `--`.
 - A few other trivial modifications (first 3 commits: 55b1a53, 4297f61, 4203079)