libtracker-miner: Do not recurse into mount points unless configured

Make TrackerFileNotifier stop recursing at folders that are mountpoints,
this makes all files indexed in a TrackerIndexingTree root be implicitly
from the same mount point.

If there are mount points in recursively indexed folders that any user
wants indexed, these will have to be configured as indexed folders
themselves.

Closes: https://gitlab.gnome.org/GNOME/tracker/-/issues/85