lib/config: Add file inside folder marker directory (#9525)

### Purpose

Avoid the issue where the folder marker is deleted by overzealous
cleanup tools because it's just a useless, empty directory.

We create a small file containing a an admonishment to not delete the
directory, and some metadata that is just for human consumption at the
moment. (But it would parse as a valid yaml file if we wanted to read
this, at some point.)

This will only apply when _creating_ a folder marker, that is, existing
setups will not gain the file automatically. Obviously, when using a
custom folder marker none of this applies.

Also, slightly adjust the permission bits for the folder marker directory and file on Unixes, making sure the group & write bits are unset.

### Testing

I've created and deleted a few folders and it appears to behave as I
expect.

### Screenshots

```
jb@ok:~/somefolder % ls -la
total 0
drwxr-xr-x   3 jb  staff   96 May  1 08:52 ./
drwx------  12 jb  staff  384 May  1 08:52 ../
drwxr-xr-x   3 jb  staff   96 May  1 08:52 .stfolder/
jb@ok:~/somefolder % ls -l .stfolder
total 8
-rw-r--r--  1 jb  staff  122 May  1 08:52 syncthing-folder-39a4b0.txt
jb@ok:~/somefolder % cat .stfolder/syncthing-folder-39a4b0.txt
# This directory is a Syncthing folder marker.
# Do not delete.

folderID: xtdca-cudyf
created: 2024-05-01T08:52:49+02:00
```

lib/model/model.go

@@ -464,9 +464,9 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 	if isPathUnique {
 		// Remove (if empty and removable) or move away (if non-empty or
 		// otherwise not removable) Syncthing-specific marker files.
-		fs := cfg.Filesystem(nil)
-		if err := fs.Remove(config.DefaultMarkerName); err != nil {
+		if err := cfg.RemoveMarker(); err != nil && !errors.Is(err, os.ErrNotExist) {
 			moved := config.DefaultMarkerName + time.Now().Format(".removed-20060102-150405")
+			fs := cfg.Filesystem(nil)
 			_ = fs.Rename(config.DefaultMarkerName, moved)
 		}
 	}