Fix io.Seek* deprecation errors

memmap.go

@@ -15,6 +15,7 @@ package afero
 
 import (
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"path/filepath"
@@ -232,7 +233,7 @@ func (m *MemMapFs) OpenFile(name string, flag int, perm os.FileMode) (File, erro
 		file = mem.NewReadOnlyFileHandle(file.(*mem.File).Data())
 	}
 	if flag&os.O_APPEND > 0 {
-		_, err = file.Seek(0, os.SEEK_END)
+		_, err = file.Seek(0, io.SeekEnd)
 		if err != nil {
 			file.Close()
 			return nil, err

memmap_test.go

@@ -223,7 +223,7 @@ func TestMultipleOpenFiles(t *testing.T) {
 		if err != nil {
 			t.Error("fs.OpenFile failed: " + err.Error())
 		}
-		_, err = fh2.Seek(0, os.SEEK_END)
+		_, err = fh2.Seek(0, io.SeekEnd)
 		if err != nil {
 			t.Error(err)
 		}

unionFile.go

@@ -47,7 +47,7 @@ func (f *UnionFile) Read(s []byte) (int, error) {
 		if (err == nil || err == io.EOF) && f.Base != nil {
 			// advance the file position also in the base file, the next
 			// call may be a write at this position (or a seek with SEEK_CUR)
-			if _, seekErr := f.Base.Seek(int64(n), os.SEEK_CUR); seekErr != nil {
+			if _, seekErr := f.Base.Seek(int64(n), io.SeekCurrent); seekErr != nil {
 				// only overwrite err in case the seek fails: we need to
 				// report an eventual io.EOF to the caller
 				err = seekErr