Add MIT kerberos tracing capability

HEIMDAL kerberos offers already tracing via a logging facility
through smb_krb5_init_context().
MIT kerberos offers to register a callback via krb5_set_trace_callback
with which tracing information can be routed to a common logging facility.
This is now integrated into smb_krb5_init_context_basic() offering
the same functionality for both kerberos fragrances.

Signed-off-by: Swen Schillig <swen@linux.ibm.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>
