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.