Change grub-mkdevicemap to emit /dev/disk/by-id/ names where

Change grub-mkdevicemap to emit /dev/disk/by-id/ names where
possible on Linux.
* util/deviceiter.c (check_device): Rename to ...
(check_device_readable_unique): ... this.  Update all callers.
Maintain and check a list of which devices (by canonicalized name)
have already been seen.
(clear_seen_devices): New function.
(compare_file_names) [__linux__]: New function.
(grub_util_iterate_devices): Clear the list of seen devices on exit
and (just in case) on entry.
(grub_util_iterate_devices) [__linux__]: Iterate over non-partition
devices in /dev/disk/by-id/, in sorted order.  Remove DM-RAID
seen-devices list, superseded by general code in check_device.