* plugin/file: shutdown reload goroutine
Shutdown the z.Reload() routine (if started in the first place) on
shutdow and reload.
Fixes#1508
* Must be put in c.OnShutdown()
* up test coverage
* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
