Unify RPC code for smart http in protocol v0/v1 and v2, which fixes
a bug in the latter (lack of authentication retry) and generally
improves the code base.

* jt/http-auth-proto-v2-fix:
  remote-curl: use post_rpc() for protocol v2 also
  remote-curl: refactor reading into rpc_state's buf
  remote-curl: reduce scope of rpc_state.result
  remote-curl: reduce scope of rpc_state.stdin_preamble
  remote-curl: reduce scope of rpc_state.argv