Config keyword cleanup.

[Hal Murray]

James Browning said:
> If it were not too late, I would suggest renaming "nts key" and "keys" to "keyfile".

I like it.  Thanks for the suggestion.

There is also "nts cookie" => cookiefile
Maybe others.

We can switch to the new names without breaking things by leaving the old 
names as aliases -- or better, aliases with a deprecated warning.  That 
would give us a handy way to test warnings during config. :)

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