Add compatibility function for older libkrb5
    
    Before krb5 1.13 the krb5_salttype_to_string() function was returning
    incorrect names (display names of some kind instead of the names
    used by the rest of the library to map saltname to the salt type
    integer number).
    This patch adds a function that checks at runtime if we have a working
    function and uses a fallback map updated to the salt types known up
    to 1.12, this allows us to use the library provided function in
    following releases where new salt types may emerge.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Milan Kubik <mkubik@redhat.com>