Comment 4 for bug 1374343

With $XDG_RUNTIME_DIR we could force exclude /run (IMHO there is no need to backup run at all). So we don't need a 'global' place

But I forgot to mention that $XDG_RUNTIME_DIR ('/run/user/<UID>') does not exist if the user is not logged in (it will be created by pam-xdg-support module during login). So even if we pass the env var to our cronjob it won't work.

1. We could also make an init script which will create /run/backintime/<USER> for all users registered on the system and make them read/writeable just by them selfs.

Pros for a global writeable folder:
- easy to force exclude to prevent backing up other users snapshots
- no root permissions necessary

Cons:
- someone else could 'occupy' the mountpoint and get in touch with users data. I don't know how this would work but I'm sure it would be possible.

2. A 'flexible' approach would make it harder to exclude. E.g. if user A would set BIT_TMP_FOLDER=/foo/bar BIT running as root wouldn't know of this and backup users A snapshots.

3. An other approach might be to place the mountpoint in '~/.local/share/backintime/mnt'. Force exclude '.local/share/backintime/mnt/' should than match all users mountpoints. We only need to make sure to clean it up propperly.