Fix seekability detection on win32.

The seeking functions on Windows internally dispatch to
 SetFilePointer(), whose behavior is undefined if you do not call
 it on "a file stored on a seeking device".
Check the type of file when it is opened and manually force seeks
 to fail if we do not have such a file.

Thanks to L W Anhonen for the report and Mark Harris for pointing
 out the solution used by opus-tools to avoid this problem.