[xmlsec] External DTD support

[Rich Salz]

> Thanks for the patch!  I applied and commited the xmlsec utility part
> with a small fix (you forgot to free DTD at the end :) ).

Great, thanks.

> And according to the OpenSSL 0.9.7 code (crypto/x509/x509_vfy.c, around
> line #200) it should work perfectly too. If you are using OpenSSL 0.9.6 
> then you might consider upgrading to 0.9.7. It'll save you a lot of time :)

We're probably going to wait for 0.9.7b before we upgrade in our 
production environment. :)

thanks for a great tool.