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/request.h

@@ -34,7 +34,7 @@ public:
     UniValue id;
     std::string strMethod;
     UniValue params;
-    enum Mode { EXECUTE, GET_HELP } mode = EXECUTE;
+    enum Mode { EXECUTE, GET_HELP, GET_ARGS } mode = EXECUTE;
     std::string URI;
     std::string authUser;
     std::string peerAddr;