boot: Deduplicate old-style loader entries.

In cases where systemd (and thus bootctl) is updated to a version
including the earlier unique-ID fix, but the corresponding new version
of systemd-boot is not installed to the ESP and run at least once,
the bootloader will report old-style entry IDs cached in the
LoaderEntries EFI variable, while bootctl will report new-style IDs for
the same entries, producing duplicate entries. This commit makes bootctl
compute and retain old-style IDs for non-auto entries so that it can
properly deduplicate entries even if the cache contains old-style IDs.