checkpoints: move the checkpoints enable boolean into main

This pertains to app-state, so it doesn't make sense to handle inside the
checkpoint functions.

src/main.h

@@ -118,6 +118,7 @@ extern int nScriptCheckThreads;
 extern bool fTxIndex;
 extern bool fIsBareMultisigStd;
 extern bool fCheckBlockIndex;
+extern bool fCheckpointsEnabled;
 extern unsigned int nCoinCacheSize;
 extern CFeeRate minRelayTxFee;