[krbdev.mit.edu #8925] [Comment] qualify_shortname default can be harmful in LAN setups

https://krbdev.mit.edu/rt/Ticket/Display.html?id=8925
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* krb5_get_credentials() ordinarily handles both checking the cache and storing
into the cache. For S4U2Self requests, it calls k5_get_proxy_cred_from_kdc(),
which stores into the cache but does not check the cache, so repeated
krb5_get_credentials() S4U2Self calls will result in duplicate cache entries.
(GSSAPI does its own cache check before making the S4U2Proxy request, and kvno
-P uses the krb5_get_credentials_for_proxy() wrapper which does a cache check.
So this is purely an issue with the krb5_get_credentials() API.)