os, syscall: implement ReadAt for unix

Windows will take more work, so test is skipped there.

src/os/file_windows.go

@@ -22,3 +22,11 @@ func Pipe() (r *File, w *File, err error) {
 	}
 	return
 }
+
+// ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
+// It returns the number of bytes read and any error encountered, possibly io.EOF.
+// At end of file, Pread returns 0, io.EOF.
+// TODO: move to file_anyos once ReadAt is implemented for windows
+func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
+	return -1, ErrNotImplemented
+}