stream: separate posix/win32 cancellation code

This code evolved into an ifdef mess as support for cancellation on
Windows was added. Make the Windows-specific code completely separate.
It looks cleaner, and it also means that some of the posix code is not
uselessly enabled on Windows. The latter made msvcrt.dll output warnings
because it does not like -1 passed as FD to read/write. (The same would
be harmless on POSIX.)