Slice 1 of #3202. The listen set, the per-name TLS attrs and the
security headers move out of vhosts.nix into ./vhost-lib.nix and are
published as `services.hyperhive.gateway.lib` (internal, readOnly).
No behaviour change: vhosts.nix consumes the published value, so the
rendered vhost tree is identical.
The point is the next slice. Today a swarm service's vhost lives in
the gateway because only the gateway knows the port pair, the issuer
for a name, and the header block. Publishing those three is what lets
a service module declare its own vhost without the gateway having to
know that service by name.
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