validation: call InvalidBlockFound also from AcceptBlock

When a block it found invalid during acceptance (but before connection)
we now mark its descendants with BLOCK_FAILED_CHILD and update
m_best_header when these things weren't done reliably before.

This does not fix a serious bug because the flags and m_best_header were being set on a best-effort basis before
and not used for anything critical.
Setting these reliably has a slight performance cost (iterating over the
entire block index) but leads to more consistency in validation and allows removing m_failed_blocks in a later commit.
This can only be triggered by providing a block with sufficient PoW
that is otherwise invalid, so it is not a DoS vector.

src/validation.cpp

@@ -4537,9 +4537,8 @@ bool ChainstateManager::AcceptBlock(const std::shared_ptr<const CBlock>& pblock,
 
     if (!CheckBlock(block, state, params.GetConsensus()) ||
         !ContextualCheckBlock(block, state, *this, pindex->pprev)) {
-        if (state.IsInvalid() && state.GetResult() != BlockValidationResult::BLOCK_MUTATED) {
+        if (Assume(state.IsInvalid())) {
-            pindex->nStatus |= BLOCK_FAILED_VALID;
+            ActiveChainstate().InvalidBlockFound(pindex, state);
-            m_blockman.m_dirty_blockindex.insert(pindex);
         }
         LogError("%s: %s\n", __func__, state.ToString());
         return false;