* guix/scripts/package.scm (switch-to-previous-generation): New function.
(roll-back): Use the new function instead of 'switch-link'.
(show-help): Add '--delete-generations'.
(%options): Likewise.
(guix-package)[process-actions]: Add 'current-generation-number',
'display-and-delete', and 'delete-generation'. Add support for
'--delete-generations', and reindent the code.
* tests/guix-package.sh: Test '--delete-generations'.
* doc/guix.texi (Invoking guix-package): Document '--delete-generations'.