Update to SSHFP tutorial

I have updated yesterday's article on how to get SSHFP records, DNSSEC, and VerifyHostKeyDNS=yes to work.

I have re-ordered the sections to avoid interrupting the flow of the instructions with chunks of background discussion.

I have also added a section discussing the improved usability vs weakened security of the RRSET_FORCE_EDNS0 patch in Debian and Ubuntu.