C++20 modernization: nullptr, constexpr, smart pointers, thread safety, enum class

- Replace ~320 NULL occurrences with nullptr across 47 files (-184 net lines)
- static const -> constexpr for version, coin, utility constants
- Collapse 9 PushMessage overloads into 1 variadic template with fold expressions
- Convert boost::array -> std::array, boost::type_traits -> std:: equivalents
- pwalletMain, pScriptCheckQueue, pScriptCheckThreads -> unique_ptr
- mapOrphanBlocks values: raw CBlock* -> unique_ptr<CBlock>
- Fix data races: add locks to wallet registration, pindexBest reads, mempool exists
- pwalletdbEncryption: raw new/delete -> local unique_ptr, remove exit() calls
- PoS reward overflow: CBigNum intermediate for nCoinAge * nRewardCoinYear
- memset -> OPENSSL_cleanse for secure zeroing
- Fix const-cast UB in SetMerkleBranch
- Log silent catch(...) blocks instead of silently swallowing
- Enum class: GetMinFeeMode, WalletFeature
- std::string_view for 8 utility function parameters
- Range-for with structured bindings: 63 iterator loops modernized
- std::make_pair -> brace init: 35 sites
- Delegating constructors: CWallet, CBlockIndex
- Merkle tree caching, std::array for GetMedianTimePast
- CScript copy ctor -> = default, operator!= -> = default

src/netbase.cpp

@@ -42,7 +42,7 @@ void SplitHostPort(std::string in, int &portOut, std::string &hostOut) {
     bool fBracketed = fHaveColon && (in[0]=='[' && in[colon-1]==']'); // if there is a colon, and in[0]=='[', colon is not 0, so in[colon-1] is safe
     bool fMultiColon = fHaveColon && (in.find_last_of(':',colon-1) != in.npos);
     if (fHaveColon && (colon==0 || fBracketed || !fMultiColon)) {
-        char *endp = NULL;
+        char *endp = nullptr;
         int n = strtol(in.c_str() + colon + 1, &endp, 10);
         if (endp && *endp == 0 && n >= 0) {
             in = in.substr(0, colon);
@@ -88,13 +88,13 @@ bool static LookupIntern(const char *pszName, std::vector<CNetAddr>& vIP, unsign
 #  endif
     aiHint.ai_flags = fAllowLookup ? AI_ADDRCONFIG : AI_NUMERICHOST;
 #endif
-    struct addrinfo *aiRes = NULL;
-    int nErr = getaddrinfo(pszName, NULL, &aiHint, &aiRes);
+    struct addrinfo *aiRes = nullptr;
+    int nErr = getaddrinfo(pszName, nullptr, &aiHint, &aiRes);
     if (nErr)
         return false;
 
     struct addrinfo *aiTrav = aiRes;
-    while (aiTrav != NULL && (nMaxSolutions == 0 || vIP.size() < nMaxSolutions))
+    while (aiTrav != nullptr && (nMaxSolutions == 0 || vIP.size() < nMaxSolutions))
     {
         if (aiTrav->ai_family == AF_INET)
         {
@@ -384,7 +384,7 @@ bool static ConnectSocketDirectly(const CService &addrConnect, SOCKET& hSocketRe
             fd_set fdset;
             FD_ZERO(&fdset);
             FD_SET(hSocket, &fdset);
-            int nRet = select(hSocket + 1, NULL, &fdset, NULL, &timeout);
+            int nRet = select(hSocket + 1, nullptr, &fdset, nullptr, &timeout);
             if (nRet == 0)
             {
                 printf("connection timeout\n");
@@ -454,7 +454,7 @@ bool SetProxy(enum Network net, CService addrProxy, int nSocksVersion) {
     if (nSocksVersion != 0 && !addrProxy.IsValid())
         return false;
     LOCK(cs_proxyInfos);
-    proxyInfo[net] = std::make_pair(addrProxy, nSocksVersion);
+    proxyInfo[net] = {addrProxy, nSocksVersion};
     return true;
 }
 
@@ -473,7 +473,7 @@ bool SetNameProxy(CService addrProxy, int nSocksVersion) {
     if (nSocksVersion != 0 && !addrProxy.IsValid())
         return false;
     LOCK(cs_proxyInfos);
-    nameproxyInfo = std::make_pair(addrProxy, nSocksVersion);
+    nameproxyInfo = {addrProxy, nSocksVersion};
     return true;
 }
 
@@ -868,7 +868,7 @@ std::string CNetAddr::ToStringIP() const
         unsigned char sha3hash[32];
         unsigned int sha3len = 0;
         EVP_MD_CTX *mdctx = EVP_MD_CTX_new();
-        EVP_DigestInit_ex(mdctx, EVP_sha3_256(), NULL);
+        EVP_DigestInit_ex(mdctx, EVP_sha3_256(), nullptr);
         EVP_DigestUpdate(mdctx, checksumInput, 48);
         EVP_DigestFinal_ex(mdctx, sha3hash, &sha3len);
         EVP_MD_CTX_free(mdctx);
@@ -890,7 +890,7 @@ std::string CNetAddr::ToStringIP() const
     socklen_t socklen = sizeof(sockaddr);
     if (serv.GetSockAddr((struct sockaddr*)&sockaddr, &socklen)) {
         char name[1025] = "";
-        if (!getnameinfo((const struct sockaddr*)&sockaddr, socklen, name, sizeof(name), NULL, 0, NI_NUMERICHOST))
+        if (!getnameinfo((const struct sockaddr*)&sockaddr, socklen, name, sizeof(name), nullptr, 0, NI_NUMERICHOST))
             return std::string(name);
     }
     if (IsIPv4())
@@ -1043,7 +1043,7 @@ static const int NET_UNKNOWN = NET_MAX + 0;
 static const int NET_TEREDO  = NET_MAX + 1;
 int static GetExtNetwork(const CNetAddr *addr)
 {
-    if (addr == NULL)
+    if (addr == nullptr)
         return NET_UNKNOWN;
     if (addr->IsRFC4380())
         return NET_TEREDO;