libnative/io: generic retry() for Unix 64 bit read/write().

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.
Note: 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 a type parameter and clean up some module imports.
auto merge of #17020 : nodakai/rust/libnative-c_int, r=alexcrichton

The current implementation of `retry()` cannot handle, say, `write()` of >2 GB data because it truncates the return value of the closure parameter into `c_int` which is (almost always) only 32 bit.

This is my first code to use traits... I hope `#[inline]` works as intended.

I think `mkerr_libc()` is still causing bugs here and there;  I'll fix them later.