Enable Windows CI (Appveyor)

- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility

plugins/ipam/host-local/backend/disk/lock.go

@@ -1,3 +1,4 @@
+// +build !windows
 // Copyright 2015 CNI authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");