unix/moduselect: If file object passed to .register(), return it in .poll().
This makes unix "uselect" compatible with baremetal "uselect". Previosuly, unix version accepted file/socket objects, but internally converted that to file descriptors, and that's what .poll() returned. To acheive new behavior, file-like objects are stored internally in an array, in addition to existing array of struct pollfd. This array is created only on first case of file-like object being passed to .register(). If only raw fd's are passed, there will be no additional memory used comparing to the original implementation.