Allow result submission to origin daemon with self-select

With `self-select` mode enabled, the `submit-to-origin` config option
will let the `SelfSelectClient` submit the solution to both
the daemon where it got the template from as well as to
the connected pool, for miners that want to do pool minining
with Monero and solo mining with an altcoin (merged mining variant).

Thank you and special credit to @StriderDM (https://github.com/StriderDM)!

src/core/config/usage.h

@@ -64,6 +64,7 @@ static inline const std::string &usage()
     u += "      --daemon                  use daemon RPC instead of pool for solo mining\n";
     u += "      --daemon-poll-interval=N  daemon poll interval in milliseconds (default: 1000)\n";
     u += "      --self-select=URL         self-select block templates from URL\n";
+    u += "      --submit-to-origin        also submit solution back to self-select URL\n";
 #   endif
 
     u += "  -r, --retries=N               number of times to retry before switch to backup server (default: 5)\n";