`host {a,aaaa}_add` has no `-ttl` option

[pederhan]

host srv_add has a -ttl option, but the more commonly used host {a,aaaa}_add commands do not. We should implement this so that we can go from:

host a_add foo 1.2.3.4
host ttl_set foo 600

To:

host a_add foo 1.2.3.4 -ttl 600

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It's unclear yet whether or not we can set TTL when creating A/AAAA records, but either way, we should strive to support setting TTL directly in the command regardless of how it's implemented on the server.