Rename "system-options" to "systemd-efi-options"

Rename "system-options" to "systemd-efi-options"

This makes the naming more consistent: we now have
bootctl systemd-efi-options,
$SYSTEMD_EFI_OPTIONS
and the SystemdOptions EFI variable.

(SystemdEFIOptions would be redundant, because it is only used in the context
of efivars, and users don't interact with that name directly.)

bootctl is adjusted to use 2sp indentation, similarly to systemctl and other
programs.

Remove the prefix with the old name from 'bootctl systemd-efi-options' output,
since it's redundant and we don't want the old name anyway.