add several backend pools to allocate mbufs clusters of various sizes out

of. currently limited to MCLBYTES (2048 bytes) and 4096 bytes until pools
can allocate objects of sizes greater than PAGESIZE.

this allows drivers to ask for "jumbo" packets to fill rx rings with.

the second half of this change is per interface mbuf cluster allocator
statistics. drivers can use the new interface (MCLGETI), which will use
these stats to selectively fail allocations based on demand for mbufs. if
the driver isnt rapidly consuming rx mbufs, we dont allow it to allocate
many to put on its rx ring.

drivers require modifications to take advantage of both the new allocation
semantic and large clusters.

this was written and developed with deraadt@ over the last two days
ok deraadt@ claudio@

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