Symbiosis: Update dns.rb to make default dmarc policy safer
Imported from https://www.github.com/BytemarkHosting/symbiosis/issues/130
The current dmarc policy is dangerous, in that it says to quarantine 100% of unaligned email from the main domain (but oddly not subdomains). This policy is much safer for an initial dmarc policy, as recommended for example, by Google at https://support.google.com/a/answer/2466563?hl=en&ref_topic=2759254
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