heimdal:kdc: Only check for default salt for des-cbc-crc enctype

heimdal:kdc: Only check for default salt for des-cbc-crc enctype

Previously, this algorithm was preferring RC4 over AES for machine
accounts in the preauth case. This is because AES keys for machine
accounts in Active Directory use a non-default salt, while RC4 keys do
not use a salt. To avoid this behaviour, only prefer keys with default
salt for the des-cbc-crc enctype.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14642
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14864

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>