krb5_k_make_checksum with 0 cksumtipe uses mandatory cksum type

[Sam Hartman]

I've been annoyed once too many times by the fact that I have a key  and I want a checksum.
That should be one call; I should not have to call krb5int_c_mandatory_cksumtype and then check the return value.

So, I'm going to treat 0 cksumtype for krb5_k_make_cheksum as "use the mandatory cksumtype for this key."

This message serves as a call for objections.