[krbdev.mit.edu #8496] Add KDC discovery from URI records

[Greg Hudson via RT]

We would like to make it possible to discover MS-KKDCP KDCs and kpasswd 
servers.  After much discussion, we think it would be best to query a URI 
record for krb5 server locations, and fall back to SRV lookups if it 
doesn't exist.

See also:

  https://www.ietf.org/mail-archive/web/kitten/current/msg06044.html
  http://mailman.mit.edu/pipermail/krbdev/2016-May/012588.html
  https://tools.ietf.org/html/draft-mccallum-kitten-krb-service-discovery

(The krbdev discussion continues in June and July.)