Reset key-generation parameters for each enctype

In add_key_pwd, initialize s2k_params to NULL inside the loop over
enctypes instead of outside the loop, so that if the afs3 salt type
is used it does not contaminate later enctype/salt pairs in the list.

ticket: 7733
tags: pullup
target_version: 1.12
Add tests for different salt combinations

Create a principal with a pair of enctypes using different salt types.
Confirm that the non-default salt type appears only once in the principal's
key list.

Also verify that the afs3 salt type is rejected by non-DES enctypes
The afs3 salt type is for compatibility with AFS-3 kaservers, which
are roughly krb4.  As such, it only makes sense for single-DES
enctypes.  The PBKDF2 and arcfour enctypes correctly reject the
key-creation parameters from the afs3 salt, but triple-DES currently
does not.