It's behind a new temporary config flag.
Also, check if the CN is over 64 bytes.
This also makes sure the certificate's Subject is not empty if the CN is
empty by always setting the SerialNumber in Subject.
While I was here, I also corrected the logged hex encoding of
SerialNumber so that its prefixed by zeroes correctly. See the use of
core.SerialToString in IssueCertificate.
I also added a test for the no CommonName and no SANs case.
Fixes#40
Jeff Hodges