Remove pointer cast in CRPCTable::dumpArgMap

CRPCTable::dumpArgMap currently works by casting RPC command unique_id
integer field to a function pointer, and then calling the function. The
unique_id field wasn't supposed to be used this way (it's meant to be
used to detect RPC aliases), and this code segfaults in the rpc_help.py
test in multiprocess PR https://github.com/bitcoin/bitcoin/pull/10102
because wallet RPC functions aren't directly accessible from the node
process.

Fix this by adding a new GET_ARGS request mode to retrieve argument
information similar to the way the GET_HELP mode retrieves help
information.

src/rpc/util.cpp

@@ -478,6 +478,9 @@ std::string RPCExamples::ToDescriptionString() const
 
 UniValue RPCHelpMan::HandleRequest(const JSONRPCRequest& request)
 {
+    if (request.mode == JSONRPCRequest::GET_ARGS) {
+        return GetArgMap();
+    }
     /*
      * Check if the given request is valid according to this command or if
      * the user is asking for help information, and throw help when appropriate.
@@ -561,8 +564,9 @@ std::string RPCHelpMan::ToString() const
     return ret;
 }
 
-void RPCHelpMan::AppendArgMap(UniValue& arr) const
+UniValue RPCHelpMan::GetArgMap() const
 {
+    UniValue arr{UniValue::VARR};
     for (int i{0}; i < int(m_args.size()); ++i) {
         const auto& arg = m_args.at(i);
         std::vector<std::string> arg_names;
@@ -577,6 +581,7 @@ void RPCHelpMan::AppendArgMap(UniValue& arr) const
             arr.push_back(map);
         }
     }
+    return arr;
 }
 
 std::string RPCArg::GetFirstName() const