Tune support for removable media in da driver:  - remove DA_FLAG_SAW_MEDIA flag, almost opposite to DA_FLAG_PACK_INVALID, using the last instead.  - allow opening device with no media present, reporting zero media size and non-zero sector size, as geom/notes suggests.  That allow to read device attributes and potentially do other things, not related to media.
Tune support for removable media in da driver:
 - remove DA_FLAG_SAW_MEDIA flag, almost opposite to DA_FLAG_PACK_INVALID,
using the last instead.
 - allow opening device with no media present, reporting zero media size
and non-zero sector size, as geom/notes suggests.  That allow to read
device attributes and potentially do other things, not related to media.

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