signet: do not log signet startup messages for other chains

and move signet network magic logging from chainparams.cpp to init.cpp

src/init.cpp

@@ -37,6 +37,7 @@
 #include <policy/fees.h>
 #include <policy/policy.h>
 #include <policy/settings.h>
+#include <protocol.h>
 #include <rpc/blockchain.h>
 #include <rpc/register.h>
 #include <rpc/server.h>
@@ -966,6 +967,9 @@ bool AppInitParameterInteraction(const ArgsManager& args)
     // specified in default section of config file, but not overridden
     // on the command line or in this network's section of the config file.
     std::string network = args.GetChainName();
+    if (network == CBaseChainParams::SIGNET) {
+        LogPrintf("Signet derived magic (message start): %s\n", HexStr(chainparams.MessageStart()));
+    }
     bilingual_str errors;
     for (const auto& arg : args.GetUnsuitableSectionOnlyArgs()) {
         errors += strprintf(_("Config setting for %s only applied on %s network when in [%s] section.") + Untranslated("\n"), arg, network, network);