Partial implementation of \h (horizontal line drawing function).

Partial implementation of \h (horizontal line drawing function).
A full implementation would require access to output device properties
and state variables (both only available after the main parser has
finalized the parse tree) before numerical expansions in the roff
preprocessor (i.e., before the main parser is even started).

Not trying to pull that stunt right now because the static-width
implementation committed here is sufficient for tcl-style manual pages
and already more complicated than i would have suspected.