[wallet] Rename InitLoadWallet() to OpenWallets()

Rationale:
- this init function can now open multiple wallets (hence
  Wallet->Wallets)
- This is named as the antonym to CloseWallets(), which carries out the
  opposite action.

src/wallet/init.cpp

@@ -228,7 +228,7 @@ bool WalletVerify()
     return true;
 }
 
-bool InitLoadWallet()
+bool OpenWallets()
 {
     if (gArgs.GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {
         LogPrintf("Wallet disabled!\n");