Add rewind logic to deal with post-fork software updates

Includes logic for dealing with pruning by Suhas Daftuar.

src/init.cpp

@@ -1422,6 +1422,14 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
                     break;
                 }
 
+                if (!fReindex) {
+                    uiInterface.InitMessage(_("Rewinding blocks..."));
+                    if (!RewindBlockIndex(chainparams)) {
+                        strLoadError = _("Unable to rewind the database to a pre-fork state. You will need to redownload the blockchain");
+                        break;
+                    }
+                }
+
                 uiInterface.InitMessage(_("Verifying blocks..."));
                 if (fHavePruned && GetArg("-checkblocks", 288) > MIN_BLOCKS_TO_KEEP) {
                     LogPrintf("Prune: pruned datadir may not have more than %d blocks; -checkblocks=%d may fail\n",