Eliminate the global 'sbbs' here, was masking bugs

I'm not sure what purpose this global variable once had, but it wasn't needed
now and it was masking copy/paste issues in some of the sbbs_t methods in this
file. Just make an 'sbbs' local to bbs_thread(), since there's where it's
legit-used.

FYI: All the Cryptlib-SSH macros assume there's an 'sbbs' in scope.