lib/model, lib/versioner: Prevent symlink attack via versioning (fixes #4286)

Prior to this, the following is possible:

- Create a symlink "foo -> /somewhere", it gets synced
- Delete "foo", it gets versioned
- Create "foo/bar", it gets synced
- Delete "foo/bar", it gets versioned in "/somewhere/bar"

With this change, versioners should never version symlinks.

lib/model/model_test.go

@@ -350,6 +350,24 @@ func (f *fakeConnection) addFile(name string, flags uint32, ftype protocol.FileI
 	f.fileData[name] = data
 }
 
+func (f *fakeConnection) deleteFile(name string) {
+	f.mut.Lock()
+	defer f.mut.Unlock()
+
+	for i, fi := range f.files {
+		if fi.Name == name {
+			fi.Deleted = true
+			fi.ModifiedS = time.Now().Unix()
+			fi.Version = fi.Version.Update(f.id.Short())
+			fi.Sequence = time.Now().UnixNano()
+			fi.Blocks = nil
+
+			f.files = append(append(f.files[:i], f.files[i+1:]...), fi)
+			return
+		}
+	}
+}
+
 func (f *fakeConnection) sendIndexUpdate() {
 	f.model.IndexUpdate(f.id, f.folder, f.files)
 }