Merge of rmind-uvmplock branch

[Mindaugas Rasiukevicius <rmind%netbsd.org@localhost>]

Hello,

Unless anyone objects, I will merge rmind-uvmplock branch.  The technical
objectives of the branch are described here:

http://nxr.netbsd.org/xref/src/doc/BRANCHES?r=1.307#488

This results in:

- Rewritten x86 TLB handler, which is simpler and cleaner C code.

- UVM provides extra serialisation for pmap(9), therefore easier to make
  implementations concurrent on other architectures, like sparc64.

- Reduced locking overhead and coalesced TLB shootdowns on x86, therefore
  some performance improvement on such workloads like ./build.sh -j N.

- Unified /dev/mem et al, and overall changes cut ~3600 lines of code.

Thanks to Andrew Doran and Joerg Sonnenberger, as their initial patches
formed the core changes of this branch.

-- 
Mindaugas