Add os stubs required for net/http

src/os/file.go

@@ -116,6 +116,11 @@ func (f *File) Readdirnames(n int) (names []string, err error) {
 	return nil, &PathError{"readdirnames", f.name, ErrNotImplemented}
 }
 
+// Seek is a stub, not yet implemented
+func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
+	return 0, &PathError{"seek", f.name, ErrNotImplemented}
+}
+
 // Stat is a stub, not yet implemented
 func (f *File) Stat() (FileInfo, error) {
 	return nil, &PathError{"stat", f.name, ErrNotImplemented}