use EXIT_ codes instead of magic numbers

To be consistent with other exit() calls.

src/bitcoind.cpp

@@ -136,17 +136,17 @@ bool AppInit(int argc, char* argv[])
         if (!AppInitBasicSetup())
         {
             // InitError will have been called with detailed error, which ends up on console
-            exit(1);
+            exit(EXIT_FAILURE);
         }
         if (!AppInitParameterInteraction())
         {
             // InitError will have been called with detailed error, which ends up on console
-            exit(1);
+            exit(EXIT_FAILURE);
         }
         if (!AppInitSanityChecks())
         {
             // InitError will have been called with detailed error, which ends up on console
-            exit(1);
+            exit(EXIT_FAILURE);
         }
         if (GetBoolArg("-daemon", false))
         {

src/test/test_bitcoin.cpp

@@ -156,12 +156,12 @@ CTxMemPoolEntry TestMemPoolEntryHelper::FromTx(const CTransaction &txn, CTxMemPo
 
 void Shutdown(void* parg)
 {
-  exit(0);
+  exit(EXIT_SUCCESS);
 }
 
 void StartShutdown()
 {
-  exit(0);
+  exit(EXIT_SUCCESS);
 }
 
 bool ShutdownRequested()