krb5: Add helper to destroy ccache as user
    
    This function safely destroy a ccache given a cache name and user crdentials.
    It becomes the user so no possible races can compromise the system, then
    uses libkrb5 functions to properly destroy a ccache, independently of the
    cache type.
    Finally restores the original credentials after closing the ccache handlers.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2061