bootctl: clean up handling of files with no version information

bootctl: clean up handling of files with no version information

get_file_version() would return:
- various negative errors if the file could not be accessed or if it was not a
  regular file
- 0/NULL if the file was too small
- -ESRCH or -EINVAL if the file did not contain the marker
- -ENOMEM or permissions errors
-  1 if the marker was found

bootctl status iterates over /EFI/{systemd,BOOT}/*.efi and checks if the files
contain a systemd-boot version tag. Resource or permission errors should be
fatal, but lack of version information should be silently ignored.

OTOH, when updating or installing bootloader files, the version is expected
to be present.

get_file_version() is changed to return -ESRCH if the version is unavailable,
and other errnos for permission or resource errors.

The logging is reworked to always display an error if encountered, but also
to log the status at debug level what the result of the version inquiry is.
This makes it figure out what is going on:
  /efi/EFI/systemd/systemd-bootx64.efi: EFI binary LoaderInfo marker: "systemd-boot 253-6.fc38"
  /efi/EFI/BOOT/BOOTfbx64.efi: EFI binary has no LoaderInfo marker.
  /efi/EFI/BOOT/BOOTIA32.EFI: EFI binary has no LoaderInfo marker.
  /efi/EFI/BOOT/BOOTX64.EFI: EFI binary LoaderInfo marker: "systemd-boot 253-6.fc38"

Replaces #27034.
Fixes https://github.com/NixOS/nixpkgs/issues/223579.