o Minor features (security):
   - Clear keys and key-derived material left on the stack in
     rendservice.c and rendclient.c. This should make us more
     forward-secure against cold-boot attacks and the like. Fix for
     bug 2385.

   - Check return value of crypto_pk_write_private_key_to_string() in
     end_service_load_keys(). This should make us more forward-secure
     against cold-boot attacks and the like. Fix for bug 2385.