0024259: Clean up OSD_File implementation

Field myFileChannel removed from OSD_FileNode and introduced to OSD_File as Linux-only along with Windows-only myFileHandle.
Implementation of OSD_File cleaned from redundant type casts; some missing type cases added.
File mode passed to fdopen() corrected in read-write case ("rw" replaced by "w+").
Method OSD_File::Size() now returns Standard_Size to be able to return correct value for large files (on 64-bit platforms only).

src/OSD/OSD_FileNode.cdl

@@ -130,7 +130,6 @@ is
 
  fields
     myPath              : Path is protected;   -- system independent path name
-    myFileChannel       : Integer is protected; -- file descriptor
     myError             : Error is protected;
 end FileNode from OSD;