Fix aklog warnings when building with Heimdal

Fix aklog warnings when building with Heimdal

Fix various warnings in the aklog and klog code when building with
Heimdal.  Use krb5_get_init_creds_opt_alloc if it's available rather
than allocating the struct off the stack, since the _opt_init
function is deprecated.

Change-Id: Ieff986121de9078e9402b0a51a8855655330caba
Reviewed-on: http://gerrit.openafs.org/2082
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>