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/vfsflags_non_unix.go

@@ -0,0 +1,18 @@
+//go:build !linux && !darwin && !freebsd
+
+package vfscommon
+
+// get the current umask
+func getUmask() int {
+	return 0000
+}
+
+// get the current uid
+func getUID() uint32 {
+	return ^uint32(0) // these values instruct WinFSP-FUSE to use the current user
+}
+
+// get the current gid
+func getGID() uint32 {
+	return ^uint32(0) // these values instruct WinFSP-FUSE to use the current user
+}