Log Message:
intel-backlight: don't set SETUID_ROOT_PERMS when UNPRIVILEGED

Requested by nia@.
In unprivileged installations the binary becomes setuid to the
unprivileged user so it doesn't work even when ran as root via sudo,
due to privilege dropping upon execution.

XXX: Arguably this fix is wrong. It doesn't make sense to me that pkgsrc
infrastructure uses mode 4511 in SETUID_ROOT_PERMS when UNPRIVILEGED.