MFC r314906: Add initial support for UNMAP granularity.

Report UNMAP granularity as stripesize/-offset if we have no other values
to report there.

Add new quirk DA_Q_STRICT_UNMAP for cases when target is too critical to
misaligned UNMAP request, reporting errors instead of being suboptimal.
Setting this quirk makes da periph to forcefully align all UNMAP requests
to avoid those errors by the cost of some odd ranges not being UNMAP'ed.
This makes UNMAP usable within VMware 6.x VMs, just now 100% efficient.