Add OnRestartFailed to the ready plugin and some various cleanups.
Document slightly better how things are supposed to work with multiple
`ready`'s in the multiple Server Blocks.
All manually tested with this Corefile:
~~~
. {
log
ready
}
example.org {
log
chaos
ready
}
~~~
And then `kill -SIGUSR1` and curling the ready endpoint. This works
well, the FailedReload is triggered by adding a syntax error in the
Corefile.
See #2659
Signed-off-by: Miek Gieben <miek@miek.nl>