[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/init.cpp

@@ -1567,7 +1567,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     // ********************************************************* Step 8: load wallet
 #ifdef ENABLE_WALLET
-    if (!InitLoadWallet())
+    if (!OpenWallets())
         return false;
 #else
     LogPrintf("No wallet support compiled in!\n");