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