added krb5 features

This request would add the ability for the nsupdate plugin to use GSS-TSIG to authenticate DNS update requests.

There are still questions: This now requires krb5-workstation even if it doesn't use it.

Is it possible to use rubygem-krb5-auth?  Looking into it.

To use TSIG, set BIND_KEYNAME and BIND_KEYVALUE
To use GSS-TSIG set BIND_KRB_PRINCIPAL and BIND_KRB_KEYTAB