lib/fs: Properly handle case insensitive systems (fixes #1787, fixes #2739, fixes #5708)

With this change we emulate a case sensitive filesystem on top of
insensitive filesystems. This means we correctly pick up case-only renames
and throw a case conflict error when there would be multiple files differing
only in case.

This safety check has a small performance hit (about 20% more filesystem
operations when scanning for changes). The new advanced folder option
`caseSensitiveFS` can be used to disable the safety checks, retaining the
previous behavior on systems known to be fully case sensitive.

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/scanner/walk.go

@@ -540,6 +540,10 @@ func (w *walker) handleError(ctx context.Context, context, path string, err erro
 	}
 }
 
+func (w *walker) String() string {
+	return fmt.Sprintf("walker/%s@%p", w.Folder, w)
+}
+
 // A byteCounter gets bytes added to it via Update() and then provides the
 // Total() and one minute moving average Rate() in bytes per second.
 type byteCounter struct {