unix: fail early on bad send_handle in uv_write2

Return UV_EBADF if send_handle doesn't have a valid fd.
unix: reduce line count, return uv__set_sys_error
unix: use uv__set_artificial_error in uv_write2

* Use uv__set_artificial_error(), slightly more efficient than
  uv__set_sys_error().

* Return UV_EINVAL instead of UV_EOPNOTSUPP.

* Fix up style.