namequery: fix get_kdc_list() to look for _kerberos records

namequery: fix get_kdc_list() to look for _kerberos records

get_kdc_list() should look for _kerberos.xxx SRV records rather
than _ldap.xxx records. This has significance in two cases:
- Non-default DNS configurations
- When building a custom krb5.conf file for a domain, an attempt is
  made to get site-specific as well as site-less records, but the
  search for _ldap records yields a cached site-specific result even
  for the site-less query.

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>