lib/config: Add file inside folder marker directory

[calmh]

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.

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