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