bitcoin-cli: remove unneeded dependencies (only minor code movement)

Remove unnecessary dependencies for bitcoin-cli
(leveldb, berkelydb, wallet, RPC server)

Build system changes:
- split `libbitcoin.a` into `libbitcoin_common.a`, `libbitcoin_server.a` and
  `libbitcoin_cli.a`

Code changes (movement only):
- split up HelpMessage into HelpMessage in init.cpp and HelpMessageCli in rpcclient.cpp
- move `uiInterface` from `init.cpp` to `util.cpp`