mdwtab.dtx: Add a sneaky kern after `\multicolumn'.

mdwtab.dtx: Add a sneaky kern after `\multicolumn'.

I don't know, any more, why `\tab@tabcr' does `\@maybe@unskip' if it's
in hmode and you ask for a nonzero interrow gap, but it does.  And the
result, if you put `\multicolumn' in the last column of a row,
terminated by `\\[...]', is that the `\tabcolsep' glue gets eaten, and,
in a right- or centre-aligned row, the cell contents ends up misaligned.

So add `\kern 0pt' to make it not do that.