update SelectParamsFromCommandLine() handling/order

- move SelectParamsFromCommandLine() from init.cpp to bitcoin.cpp to allow
  to use TestNet() for Bitcoin-Qt instead of GetBoolArg("-testnet", false)
- change order in bitcoind.cpp to match bitcoin.cpp functionality
- hamonize error message strings for missing datadir and failing
  SelectParamsFromCommandLine() in bitcoin.cpp and bitcoind.cpp
- use TestNet() call in splashscreen.cpp