Handle the GSS-TSIG key name uniqueness

Quoting RFC 2930:

At any DNS server or resolver only one octet string of keying material may be in place for any particular key name. An attempt to establish another set of keying material at a server for an existing name returns a BADNAME error.

We should be prepare to handle this case. Note I do not propose to add code because it is very unlikely (random part on 32 bits and name lookup before creating a new key) but to add an unit test so the condition can be recognized.