mdwtools.tex, *.dtx: Provide a hack for setting `description' labels.

mdwtools.tex, *.dtx: Provide a hack for setting `description' labels.

LaTeX has a bug (which I'm sure can't be fixed without unacceptable
compatibility lossage) which starts setting `description' list labels
`\labelsep' too far to the left, so the `\makelabel' implementation
compensates by adding a `\labelsep' of horizontal space before the
actual label text.  The array rule-spec list overrides `\makelabel' to
reduce typing, and therefore must duplicate this countermeasure.

My `strayman' document class, and the `mdwlist' package in this
collection, both correct this bug, so if either of those is loaded then
the list ends up with the labels too far to the right.  (This is, of
course, the very compatibility problem that prevents the bug from being
fixed upstream.)

Provide a command which detects (hackily) either of these two situations
and sets a label with the correct spacing, and use it everywhere.