Description of problem:
When generating Host Certificates it should be possible to specify that TLSA
DNS records be generated.
Additionally/Alternatively the "Show Certificate" option should list the
various likely options for TLSA records. Most pertinently it should give the
SHA256 for the full cert and for the subject rather than forcing the user to
locate the cert and perform some manual work to determine the values.
The defaults should be
  Certificate Usage - 3
  Selector - 0
  Matching Type - 1
The sort order should be improved in the GUI to ensure that service selector
components like TLSA records are located under the host record.
Possibly consider putting TLSA records in as
  _cert.host TLSA a b c nnnnnnnnnn
and providing simpler mech to create
  _port._(tcp|udp).host CNAME _cert.host
records.