* Make it so the code that generates a signed closure can access the
gateway private key if the private key was generated as part of the same
command.
* For now, only use the gateway private key to verify closure signatures
in the RG.  This removes the code that attempts to use the user private
key, and falls back to the gateway key (which is not desirable when
hundreds of separate instances are involved, since it means keeping
hundreds of copies of user private keys around).