bootctl: add @current/@oneshot/@default targets to set-default/set-oneshot

bootctl: add @current/@oneshot/@default targets to set-default/set-oneshot

Using `bootctl set-default @current` will set the default loader entry
to the currently booted entry as read from the `LoaderEntrySelected` EFI
variable.

Also `bootctl set-oneshot @current` will set the oneshot loader entry to
the current booted entry.

Correspondingly `@default` and `@oneshot` can be used to read from the
LoaderEntryDefault and LoaderEntryOneshot EFI variables.