[krbdev.mit.edu #1620] should have option to randomize salt at pw change

[Ken Raeburn via RT]

As I'm writing up in the AES draft, always using a known salt string
for a given principal allows an attacker to build up a dictionary of
keys from a password dictionary, and save intermediate results to
re-use even if the iteration count is varied when the password is
changed.

We should have an option to randomize the salt string when the
password is changed.  For AES, at least; I don't think it would do any
harm for other encryption types.

Ken