edit-chroot, spinner: define ret as a local variable
edit-chroot/backup: Delete partial archive on failure.

Ensures only complete archive is leftover.
Check that chroot names are valid:
No / in the name; name cannot be `.`, `..` or empty string.
Allow chroot name starting in `-`.

Make sure chroot names are separated from other arguments when calling `{mount,unmount,edit}-chroot`.