Add dm_device_get_name to get map name or block device name for given devno.

Add dm_device_get_name to get map name or block device name for given devno.

This is accomplished by reading associated sysfs information. For a dm device,
this is /sys/dev/block/major:minor/dm/name (supported in kernel version >= 2.6.29,
for older kernels, the behaviour is the same as for non-dm devices).

For a non-dm device, this is a readlink on /sys/dev/block/major:minor, e.g.
  /sys/dev/block/253:0 --> ../../devices/virtual/block/dm-0.
The last component of the path is a proper kernel name (block device name).

One can request to read only kernel names by setting the 'prefer_kernel_name'
argument if needed.