[krbdev.mit.edu #1583] use locate_server for krb4 kdc location

[Ken Raeburn via RT]

Fix ticket 1550 better. :-)

The krb4 lookup needs some extra hooks:

 * use "v4 realms" top-level krb5.conf section instead of "realms"

 * after krb5.conf and before dns srv, look at krb.conf (a callback
   function is probably good for this)

Then stop "exporting" (via accessor hooks) the DNS SRV code.

See also tickets 1511, 1550.