[pretest]: Adapt tlmgr message strings to w64?

[Karl Berry]

    | Create Start menu shortcuts (w32) (desktop_integration): 1
    | Change file associations (w32) (fileassocs): 0

I changed the option descriptions to say "Windows". Thanks for
noticing+reporting.

BTW, in searching for other occurrences, I saw that the tlmgr path
command also has an option named --w32mode. We'll change that at some
point, but it's more than I wanted to do right now. --thanks, karl.