Do merkle root and txid duplicates check simultaneously

Move the txid duplicates check into BuildMerkleTree, where it can be done
much more efficiently (without needing to build a full txid set to detect
duplicates).

The previous version (using the std::set<uint256> to detect duplicates) was
also slightly too weak. A block mined with actual duplicate transactions
(which is invalid, due to the inputs of the duplicated transactions being
seen as double spends) would trigger the duplicates logic, resulting in the
block not being stored on disk, and rerequested. This change fixes that by
only triggering in the case of duplicated transactions that can actually
result in an identical merkle root.
minor variable init changes in init.cpp

- explicit init of pcoinsdbview and pwalletMain (even if not needed, as
  globals are init to NULL, it seems cleaner)
- remove check if (pwalletMain) in Shutdown() as delete is valid even if
  pwalletMain is NULL
Keep symlinks when copying into .app bundle

Code signing failed for me on OSX 10.9.5 because the
Versions/Current symbolic links were being replaced
with a duplicate copy of the frameworks' code.

Releases were bigger than they needed to be, for the
same reason.
Merge pull request #5024 from gavinandresen/osx_sign_fix

Keep symlinks when copying into .app bundle
Merge pull request #4980

20a11ff minor variable init changes in init.cpp (Philip Kaufmann)
Merge pull request #4926

584a358 Do merkle root and txid duplicates check simultaneously (Pieter Wuille)
Headers-first synchronization

Many changes:
* Do not use 'getblocks', but 'getheaders', and use it to build a headers tree.
* Blocks are fetched in parallel from all available outbound peers, using a
  limited moving window. When one peer stalls the movement of the window, it is
  disconnected.
* No more orphan blocks. At all. We only ever request a block for which we have
  verified the headers, and store it to disk immediately. This means that a
  disk-fill attack would require PoW.
* Require protocol version 31800 for every peer (released in december 2010).
* No more syncnode (we sync from everyone we can, though limited to 1 during
  initial *headers* sync).
* Introduce some extra named constants, comments and asserts.
RPC additions after headers-first
Remove CheckMinWork, as we always know all parent headers
Improve getheaders (sending) logging
Better logging of stalling
Add height to "Requesting block" debug