First, make extra data cells be thrown away. This makes "dp->layout"

First, make extra data cells be thrown away.  This makes "dp->layout"
always hold, which cleans up the table stuff a bit.

Second, set a "spans" value per data cell consisting of the number of
skipped TBL_CELL_SPAN layout cells.

Third, make tbl_term.c understand how to skip over spanned sections when
iterating over the header queue.

What remains is to calculate the widths of spanned cells.