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

### 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
```

0 files changed, 0 insertions, 0 deletions