Do text production for .Bt, .Ex, .Rv, .Ud at the validation stage

Do text production for .Bt, .Ex, .Rv, .Ud at the validation stage
rather than in the formatters.  Use NODE_NOSRC flag for .Lb and
NODE_NOSRC and NODE_NOPRT for .St.  Results in a more rigorous
syntax tree and in 135 lines less code.

This work was triggered by a question from Abhinav Upadhyay <er dot
abhinav dot upadhyay at gmail dot com> (NetBSD) on discuss@.