Add $INCLUDE support

This doesn't support the origin stacking defined in RFC 1034 because:

1. Doing so would require us to maintain a real $INCLUDE stack instead
   of just chaining iterators; and

2. The expected use case is including automatically-generated snippets
   in zones that are being maintained with zc, so there's no real need
   for origin fiddling anyway because whatever automation is
   generating the snippets can just generate FQDNs if necessary.

If really needed, we could fix this, but, YAGNI.

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