locale-util: extend comments on unicode glyph use, and drop mdash (that actually was an ndash)

Let's add the actual unicode names of the glyphs we use. Let's also add
in comments what the width expectations of these glyphs are on the
console.

Also, remove the "mdash" definition. First of all it wasn't used, but
what's worse the glyph encoded was actually an "ndash"...

Fixes: #14075