Don't make the event loop spin when connect() returns EINPROGRESS.

Test case:

  #include "uv.h"

  static void connect_cb(uv_connect_t* req, int status) {
    // ...
  }

  int main() {
    uv_tcp_t handle;
    uv_connect_t req;
    struct sockaddr_in addr;
    addr = uv_ip4_addr("8.8.8.8", 1234); // unreachable
    uv_tcp_init(uv_default_loop(), &handle);
    uv_tcp_connect(&req, (uv_stream_t*)&handle, addr, connect_cb);
    uv_run(uv_default_loop()); // busy loops until connection times out
    return 0;
  }

After EINPROGRESS, there are zero active handles and one active request. That
in turn makes uv__poll_timeout() believe that it should merely poll, not block,
in epoll() / kqueue() / port_getn().

Sidestep that by artificially starting the handle on connect() and stopping it
again once the TCP handshake completes / is rejected / times out.

It's a slightly hacky approach because I don't want to change the ABI of the
stable branch. I'll address it properly in the master branch.