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

src/init.cpp

@@ -373,9 +373,6 @@ bool AppInit2(boost::thread_group& threadGroup)
     // ********************************************************* Step 2: parameter interactions
 
     Checkpoints::fEnabled = GetBoolArg("-checkpoints", true);
-    if (!SelectParamsFromCommandLine()) {
-        return InitError("Invalid combination of -testnet and -regtest.");
-    }
 
     if (mapArgs.count("-bind")) {
         // when specifying an explicit binding address, you want to listen on it