remove win2k gss-tsig hacks
Remove the code implementing nonstardard behaviors that were formerly needed to allow GSS-TSIG to work with Windows 2000, which passed End-of-Life in 2010.
Deprecate the "oldgsstsig" command and "-o" command line option to nsupdate; these are now treated as synonyms for "gsstsig" and "-g" respectively.
Closes #4012 (closed)