By default the zpool_layout command would always use the slot
number assigned by Linux when generating the zdev.conf file.
This is a reasonable default there are cases when it makes
sense to remap the slot id assigned by Linux using your own
custom mapping.
This commit adds support to zpool_layout to provide a custom
slot mapping file. The file contains in the first column the
Linux slot it and in the second column the custom slot mapping.
By passing this map file with '-m map' to zpool_config the
mapping will be applied when generating zdev.conf.
Additionally, two sample mapping have been added which reflect
different ways to map the slots in the dragon drawers.
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