on FreeBSD. Define HAVE_DIOCGDINFO on NetBSD and FreeBSD to reduce
the verbosity of later #ifs.
(find_partition_start): Define this function on FreeBSD too.
(device_is_wholedisk) [__FreeBSD__ || __FreeBSD_kernel__]: New
function.
(grub_util_biosdisk_get_grub_dev): Use partition-start-sector logic
on FreeBSD.