IPV6 as default

remove the option to build without an ipv6 support
Remove some unused code
Relay OP_RETURN TxOut as standard transaction type
Show short scriptPubKeys correctly

Previously bitcoin-qt's -debug transaction info was showing
CTxOut([error])

It is valid for a scriptPubKey to be any size, for example simply
OP_RETURN is valid and can be used to destroy a TXOUT to mining fees.

Conflicts:
src/core.cpp
Remove broken OptionsModel::Upgrade()
remove setting methods in wallet/walletdb
Remove duplicate and unused strWalletFileName
P2P: parse network datastream into header/data components

Replaces CNode::vRecv buffer with a vector of CNetMessage's.  This
simplifies
ProcessMessages() and eliminates several redundant data copies.
datastream improvements

Use per-message send buffer, rather than per connection