fix(postfix): Fix non-functional dns lookups for lmtp

This patch fixes the unexpected dns lookup behaviour of LMTP.

By default, lmtp only check DNS names directly, by that it ignores
search domains and alike which breaks Kubernetes' lookup behaviour for
namespace internal service names, unless you explicitly configure the
whole FQDN.

This is especially confusing, because the same DNS name works perfectly
fine for SASL lookups. So while the SASL authentication works fine, the
LMTP delivery fails.

This behaviour can be fixed, by enabling native lookups, which will
re-enable regular nss behaviour including the usage of search domains,
which will allow all Kubernetes aspects to work like a charm.

I caught this on the postfix mailing list, after investigating the odd
behaviour for almost a day.

References:
https://marc.info/?l=postfix-users&m=142793569913728&w=2