New getwork

I uploaded a redesign of m0mchil's getwork to SVN rev 189 (version 31601)

m0mchil's external bitcoin miner idea has solved a lot of problems.  GPU programming is immature and hard to compile, and I didn't want to add additional dependencies to the build.  getwork allows these problems to be solved separately, with different programs for different hardware and OSes.  It's also convenient that server farms can run a single Bitcoin node and the rest only run getwork clients.

The interface has a few changes:

getwork [data]
If [data] is not specified, returns formatted hash data to work on:
  "midstate" : precomputed hash state after hashing the first half of the data
  "data" : block data
  "hash1" : formatted hash buffer for second hash
  "target" : little endian hash target
If [data] is specified, tries to solve the block and returns true if it was successful.  [data] is the same 128 byte block data that was returned in the "data" field, but with the nonce changed.

Notes:
- It does not return work when you submit a possible hit, only when called without parameter.
- The block field has been separated into data and hash1.
- data is 128 bytes, which includes the first half that's already hashed by midstate.
- hash1 is always the same, but included for convenience.
- Logging of "ThreadRPCServer method=getwork" is disabled, it would be too much junk in the log.