Ensure that certificate policies follow RFC 7318

Ensure that certificate policies follow RFC 7318

RFC 7318 makes requirements on the certificate policy extension imposed
by RFC 6487 a bit stricter. It requires that exactly one policy OID is
present and that it be id-cp-ipAddr-asNumber and if there is a policy
qualifier it must be id-qt-cps. These are requirements that the X.509
verifier's policy code can't enforce, so unpack the certificate policy
extension by hand and check that it matches expectations.

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