Always include salt in LDAP KrbKey encoding

In the LDAP KDB module, ensure that every krb5_key_data we pass to
asn1_encode_sequence_of_keys includes a salt type, for compatibility
with the decoder in unpatched krb5 1.11 and 1.12.

This is not a behavior change by itself; since 1.7 the encoder has
always included a KrbKey salt field because it erroneously treats that
field as non-optional.  (Luckily, the encoded salt always happens to
have salt type 0 because krb5_key_data constructors start with zeroed
memory.)  The next commit will fix the encoder and decoder to properly
treat the KrbKey salt field as optional, so we need this change to
ensure that our encodings remain compatible.

Also fix the ASN.1 tests to set key_data_ver correctly for the sample
test key data.

ticket: 7919
Treat LDAP KrbKey salt field as optional

Per the ASN.1 definition, the KrbKey salt field is optional.  Since
1.7, we have been treating it as mandatory in the encoder; since 1.11,
we have been treating it as mandatory in the decoder.  Mostly by luck,
we have been encoding a salt type of 0 when key_data_ver is 1, but we
really should not be looking at key_data_type[1] or key_data_length[1]
in this situation.  Treat the salt field as optional in the encoder
and decoder.  Although the previous commit ensures that we continue to
always encode a salt (without any dangerous assumptions about
krb5_key_data constructors), this change will allow us to decode key
data encoded by 1.6 without salt fields.

This also fixes issue #7918, by properly setting key_data_ver to 2 if
a salt type but no salt value is present.  It is difficult to get the
decoder to actually assign 2 to key_data_ver just because the salt
field is there, so take care of that in asn1_decode_sequence_of_keys.

Adjust kdbtest.c to match the new behavior by setting key_data_ver to
2 in both test keys.

ticket: 7919
target_version: 1.12.2
tags: pullup