libnative/io: fix a compile error on Windows due to b8911d7

The error was about the way to compare os::errno() with libc::WSAEINTR.
But Win32/WinSock APIs never call WSASetLastError() with WSAEINTR
unless a programmer specifically cancels the ongoing blocking call by
a deprecated WinSock1 API WSACancelBlockingCall().
So the errno check was simply removed and retry() became an id function
on Windows.

Windows' equivalent of SIGINT is always handled in a separate thread:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682541%28v=vs.85%29.aspx
CTRL+C and CTRL+BREAK Signals

Also, incidentally rename the type parameter.