vfs: convert vfs options to new style

This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests

vfs/vfscommon/filemode.go

@@ -0,0 +1,40 @@
+package vfscommon
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	"github.com/rclone/rclone/fs"
+)
+
+// FileMode is a command line friendly os.FileMode
+type FileMode os.FileMode
+
+// String turns FileMode into a string
+func (x FileMode) String() string {
+	return fmt.Sprintf("%03o", x)
+}
+
+// Set a FileMode
+func (x *FileMode) Set(s string) error {
+	i, err := strconv.ParseInt(s, 8, 32)
+	if err != nil {
+		return fmt.Errorf("bad FileMode - must be octal digits: %w", err)
+	}
+	*x = (FileMode)(i)
+	return nil
+}
+
+// Type of the value
+func (x FileMode) Type() string {
+	return "FileMode"
+}
+
+// UnmarshalJSON makes sure the value can be parsed as a string or integer in JSON
+func (x *FileMode) UnmarshalJSON(in []byte) error {
+	return fs.UnmarshalJSONFlag(in, x, func(i int64) error {
+		*x = FileMode(i)
+		return nil
+	})
+}