remove win2k GSS-TSIG hacks

Our GSSAPI implementation includes non-standard code to allow interoperation with Windows 2000. That OS reached its end of life 13 years ago, so it would probably be worthwhile to consider removing the extra code.

This would involve deprecating the -o option and the oldgsstsig command to nsupdate.

My suggestion is to remove the functionality now and make -o a synonym for -g and oldgsstsig a synonym for gsstsig. We can actually remove them in a later release.