* plugins: Return error for multiple use of some
Return plugin.ErrOnce when a plugin that doesn't support it, is called
mutliple times.
This now adds it for: cache, dnssec, errors, forward, hosts, nsid.
And changes it slightly in kubernetes, pprof, reload, root.
* more tests
Reloading should work (kill -TERM reload the coredns process), but a lot
of plugins can't handle it proper. Disable to reload plugin until we fix
(most) of the plugins
* tune usage of var global, add limit to options
* update readme for minimal values
* useless change to quick-off codecov
* fix msgs for min values and tune the flag for end of reload usage, with a 'maybe' option
* adding UT for min values, adding MD5 of corefile on the log
