all: Add filesystem notification support

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3986

lib/fs/basicfs_watch_unsupported.go

@@ -0,0 +1,15 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// +build solaris,!cgo darwin,!cgo
+
+package fs
+
+import "context"
+
+func (f *BasicFilesystem) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, error) {
+	return nil, ErrWatchNotSupported
+}