os-bind: RNDC key file separation

[arichtman]

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Is your feature request related to a problem? Please describe.

Presently, the Bind RNDC key is stored alongside some RNDC options in /usr/local/etc/namedb/rndc.conf.
This is incompatible with nsupdate, which can otherwise use the key in that format, but not with the options in the same file.
This makes configuring DDclient trickier.

Describe the solution you'd like

Pull the key section out into another, distinct file (probably rndc.conf.key), and then just include it in rndc.conf.

Describe alternatives you've considered

• Manually copy the key out to another file every time the secret changes.
• Make a hook script to do the above.

Additional context