* 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
* tests: CoreDNSServerAndPorts
Copy from kubernetes.go and renamed to fit the style, adapted almost
all callers.
This is a mechanicl change, no testdata was changed.
* typos
