Integrate kerberos tracing

[swen]

This is actually a resend from Nov. 26th with one small but important
addition.

It now includes an option for 'configure' with which the feature can be
selected.

This patch set is using krb5_set_trace_callback from the kerberos API
to provide inline tracing information.

The feature itself must be enabled at compile time by defining
HAVE_KRB5_TRACING, the default is to NOT have tracing compiled in.
As mentioned above, this can be done/enabled now by specifiying the
configuration option --with-krb5-tracing.

If compiled in, the trace information will only be logged if logging is
set to DEBUG level.

Beside the introduction of the feature (patch 1) the inclusion for each
area is handled by tiny seperate patches (patch 2 - 10).
If wanted, that could be condensed to one, but I remember that some
prefer it that way.
The last patch (patch 11) contains now the changes to the wscript,
offering the configuration switch.

The entire patch set passed the GitLab test suit and a merge request is
created as well.

https://gitlab.com/samba-team/samba/merge_requests/123

Please review, comment and maybe push if happy.

Cheers Swen