core: watch paths with symlinks in .path units

When watching paths that contain symlinks in some element we so far
always only watched the inode they are pointing to, not the symlink
inode itself. Let's fix that and always watch both. We do this by simply
installing the inotify watch once with and once without IN_DONT_FOLLOW.
For non-symlink inodes this just overrides the same watch twice (where
the second one replaces the first), which is has no effect effectively.
For symlinks it means we'll watch both source and destination.

Fixes: #17727