Comment use of macupdate

@vitorgalvao, @ndr-qef, the first two are cases where I would use `license :abandoned`.

It would seem prudent to print a warning in such cases.
I find it slightly more readable for monospace, but it's not a strong preference.  My editor is configured that way, so it happens without thinking.

Either convention is fine.  But it would probably save effort at this point to have a short English style guide (probably in HACKING.md?).  Naive `wc` says we have 20K words of Markdown, plus uncounted message strings and caveats.