The routine opens and searches keytab for the entry identified
by principal, keytype, and vno,
returning the resulting key in *key or
returning an error code if it is not found. If keyprocarg
is not NULL, it is taken to be a char* denoting
the name of a keytab. Otherwise, the default keytab will be used.
krb5_free_keyblock should be called
on *key when the caller is finished with