New database check routine

-checklevel gets a new meaning:
0: verify blocks can be read from disk (like before)
1: verify (contextless) block validity (like before)
2: verify undo files can be read and have good checksums
3: verify coin database is consistent with the last few blocks
   (close to level 6 before)
4: verify all validity rules of the last few blocks

Level 3 is the new default, as it's reasonably fast. As level 3 and
4 are implemented using an in-memory rollback of the database, they
are limited to as many blocks as possible without exceeding the
limits set by -dbcache. The default of -dbcache=25 allows for some
150-200 blocks to be rolled back.

In case an error is found, the application quits with a message
instructing the user to restart with -reindex. Better instructions,
and automatic recovery (when possible) or automatic reindexing are
left as future work.

src/init.cpp

@@ -300,7 +300,7 @@ std::string HelpMessage()
         "  -rescan                " + _("Rescan the block chain for missing wallet transactions") + "\n" +
         "  -salvagewallet         " + _("Attempt to recover private keys from a corrupt wallet.dat") + "\n" +
         "  -checkblocks=<n>       " + _("How many blocks to check at startup (default: 2500, 0 = all)") + "\n" +
-        "  -checklevel=<n>        " + _("How thorough the block verification is (0-6, default: 1)") + "\n" +
+        "  -checklevel=<n>        " + _("How thorough the block verification is (0-4, default: 3)") + "\n" +
         "  -loadblock=<file>      " + _("Imports blocks from external blk000??.dat file") + "\n" +
         "  -reindex               " + _("Rebuild blockchain index from current blk000??.dat files") + "\n" +
 
@@ -752,7 +752,10 @@ bool AppInit2()
         pblocktree->WriteReindexing(true);
 
     if (!LoadBlockIndex())
-        return InitError(_("Error loading blkindex.dat"));
+        return InitError(_("Error loading block/coin databases"));
+
+    if (!VerifyDB())
+        return InitError(_("Corrupted database detected. Please restart the client with -reindex."));
 
     // as LoadBlockIndex can take several minutes, it's possible the user
     // requested to kill bitcoin-qt during the last operation. If so, exit.