Move ui_interface to bitcoin_server.a

There is no need for it in the utility libraries or tools.
Put it in init.cpp, and in the tests separately (as they can't link init).

src/bitcoin-cli.cpp

@@ -7,11 +7,12 @@
 #include "init.h"
 #include "rpcclient.h"
 #include "rpcprotocol.h"
-#include "ui_interface.h" /* for _(...) */
 #include "chainparamsbase.h"
 
 #include <boost/filesystem/operations.hpp>
 
+#define _(x) std::string(x) /* Keep the _() around in case gettext or such will be used later to translate non-UI */
+
 using namespace std;
 using namespace boost;
 using namespace boost::asio;