validation: allow to specify frequency for -checkblockindex

This makes it similar to -checkaddrman and -checkmempool, which
also allow to run the check occasionally instead of always / never.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/chainstatemanager_args.cpp

@@ -24,7 +24,10 @@
 namespace node {
 util::Result<void> ApplyArgsManOptions(const ArgsManager& args, ChainstateManager::Options& opts)
 {
-    if (auto value{args.GetBoolArg("-checkblockindex")}) opts.check_block_index = *value;
+    if (auto value{args.GetIntArg("-checkblockindex")}) {
+        // Interpret bare -checkblockindex argument as 1 instead of 0.
+        opts.check_block_index = args.GetArg("-checkblockindex")->empty() ? 1 : *value;
+    }
 
     if (auto value{args.GetBoolArg("-checkpoints")}) opts.checkpoints_enabled = *value;