Compliant rebrand: legal compliance, code audit, and documentation (Phases 0-3)

Phase 0 - Legal Compliance:
- COPYING: Add DragonX copyright, preserve all upstream attributions
- AUTHORS: Add DragonX developers section
- LICENSE: Restore standard FSF GPLv3 text (fix GNU→GENERAL error)
- Add DragonX copyright headers to all 21 modified source files
- contrib/debian/copyright: Add DragonX attribution
- README.md: Add GPLv3 Section 5(a) attribution section

Phase 1 - init.cpp Cleanup:
- PID file: hushd.pid → dragonxd.pid
- Shutdown thread: hush-shutoff → dragonx-shutoff
- Debug message: stopping HUSH → stopping DragonX

Phase 2 - HUSH3/ishush3 Code Audit:
- Rename ishush3 → isdragonx across ~15 source files
- Update "HUSH3" chain-identity checks to "DRAGONX" in consensus,
  difficulty, notarization, devtax, and RPC code
- Intentionally preserve cross-chain "HUSH3" refs (gateway, notary dest)
- Build verified: all three binaries compile cleanly

Phase 3 - Documentation:
- README.md: Full rewrite with DragonX chain params, build instructions
- Man pages: Create dragonxd.1, dragonx-cli.1, dragonx-tx.1 (v1.0.0)
- Doc files: Add beefy-DRAGONX.conf, dragonxd-systemd.md, dragonxd.service
- Init scripts: Create dragonxd.{conf,init,openrc,openrcconf,service}
- Debian packaging: Update control, changelog, install, manpages, examples
- Update doc/init.md and contrib/init/README.md

src/wallet/rpcwallet.cpp

@@ -712,7 +712,7 @@ UniValue kvupdate(const UniValue& params, bool fHelp, const CPubKey& mypk)
         //for (i=0; i<32; i++)
         //    printf("%02x",((uint8_t *)&sig)[i]);
         //printf(" sig for keylen.%d + valuesize.%d\n",keylen,refvaluesize);
-        ret.push_back(Pair("coin",(char *)(SMART_CHAIN_SYMBOL[0] == 0 ? "HUSH3" : SMART_CHAIN_SYMBOL)));
+        ret.push_back(Pair("coin",(char *)(SMART_CHAIN_SYMBOL[0] == 0 ? "DRAGONX" : SMART_CHAIN_SYMBOL)));
         height = chainActive.LastTip()->GetHeight();
         if ( memcmp(&zeroes,&refpubkey,sizeof(refpubkey)) != 0 )
             ret.push_back(Pair("owner",refpubkey.GetHex()));