r6094: Work on the Kerberos code recently merged from Samba 3.0. This fixes

r6094: Work on the Kerberos code recently merged from Samba 3.0.  This fixes
up issues I introduced during the merge, that caused a segfault.

I've still not got the keytab code to work for me (using Samba3 to
generate the keytab) so this is still not fully tested, but it's
better than it was.

To add debugging, I now use the krb5_get_error_message() function from
Heimdal when present, to return the custom error string, which
contains far, far more information than the simple error code does.

(This last point may well be worth merging back into 3.0)

Andrew Bartlett