[Groff] Another namespace issue (tbl vs. hdtbl)

[Larry Kollar]

Multi-page tbl tables don't like getting hdtbl pulled in -- both of  
them use .TH and groff hangs at the first tbl usage of .TH.

Yes, there are situations where you might want to mix tbl and hdtbl  
tables in a single document -- for example, in a transition from one  
to another, or you might mostly use hdtbl but need tbl for the  
occasional multi-page table.

I think the answer for now is to rename the hdtbl usage from .TH  
to .THD or something similar -- a trivial fix in the tmac file, but  
will also require fixing the manpage and examples.

-- Larry