(grub_device_mapper_supported): New function.
* include/grub/emu/misc.h (grub_device_mapper_supported): Add
prototype.
* kern/emu/hostdisk.c (find_partition_start): Check whether
device-mapper is supported before trying to use it.
* util/deviceiter.c (grub_util_iterate_devices): Likewise.
return NULL for various reasons even if it has a maximum-length
buffer: for example, there might be a symlink loop, or the path
might exceed PATH_MAX. If this happens, return NULL.
