Save the full residual for keyring caches

krb5_cc_get_name() should allow the caller to reconstruct the full
cache name.  That is not possible if thread: and process: are omitted
here.  (The saved name is not used by anything except
krb5_krcc_get_name, so this change is safe.)

[ghudson@mit.edu: proofread and clarified commit message]

ticket: 7692 (new)