Buffer cache revamp

1) remove multiple size queues, introduced as a stopgap.
2) decouple pages containing data from their mappings
3) only keep buffers mapped when they actually have to be mapped
  (right now, this is when buffers are B_BUSY)
4) New functions to make a buffer busy, and release the busy flag
   (buf_acquire and buf_release)
5) Move high/low water marks and statistics counters into a structure
6) Add a sysctl to retrieve buffer cache statistics

Tested in several variants and beat upon by bob and art for a year. run
accidentally on henning's nfs server for a few months...

ok deraadt@, krw@, art@ - who promises to be around to deal with any fallout

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