bugfix: Streamline setting reindex option

Reverts a bug introduced in b47bd95
"kernel: De-globalize fReindex". The change leads to a GUI user being
prompted to re-index on a chainstate loading failure more than once as
well as the node actually not reindexing if the user chooses to. Fix
this by setting the reindexing option instead of the atomic, which can
be safely re-used to indicate that a reindex should be attempted.

The bug specifically is caused by the chainman, and thus the blockman
and its m_reindexing atomic being destroyed on every iteration of
the for loop.

The reindex option for ChainstateLoadOptions is currently also set in a
confusing way. By using the reindex atomic, it is not obvious in which
scenario it is true or false.

The atomic is controlled by both the user passing the -reindex option,
the user chosing to reindex if something went wrong during chainstate
loading when running the gui, and by reading the reindexing flag from
the block tree database in LoadBlockIndexDB. In practice this read is
done through the chainstate module's CompleteChainstateInitialization's
call to LoadBlockIndex. Since this is only done after the reindex option
is set already, it does not have an effect on it.

Make this clear by using the reindex option from the blockman opts which
is only controlled by the user.

Author: TheCharlatan

src/init.cpp

@@ -1558,7 +1558,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
         node::ChainstateLoadOptions options;
         options.mempool = Assert(node.mempool.get());
-        options.reindex = chainman.m_blockman.m_reindexing;
+        options.reindex = blockman_opts.reindex;
         options.reindex_chainstate = fReindexChainState;
         options.prune = chainman.m_blockman.IsPruneMode();
         options.check_blocks = args.GetIntArg("-checkblocks", DEFAULT_CHECKBLOCKS);
@@ -1600,13 +1600,13 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
         if (!fLoaded && !ShutdownRequested(node)) {
             // first suggest a reindex
-            if (!options.reindex) {
+            if (!blockman_opts.reindex) {
                 bool fRet = uiInterface.ThreadSafeQuestion(
                     error + Untranslated(".\n\n") + _("Do you want to rebuild the block database now?"),
                     error.original + ".\nPlease restart with -reindex or -reindex-chainstate to recover.",
                     "", CClientUIInterface::MSG_ERROR | CClientUIInterface::BTN_ABORT);
                 if (fRet) {
-                    chainman.m_blockman.m_reindexing = true;
+                    blockman_opts.reindex = true;
                     if (!Assert(node.shutdown)->reset()) {
                         LogPrintf("Internal error: failed to reset shutdown signal.\n");
                     }