[PATCH] wg: include filename in error messages

[Tony Finch]

Yesterday when I was setting up wireguard using ifupdown, I got a "file
not found" error which mentioned neither the program nor the filename, so
I was at a loss. After making ifupdown more verbose the problem turned out
to be wireguard.

When I was preparing this patch I really wanted to change all the error
reporting to use err(3) because in my experience it's reasonably nice for
the programmer and the user. Dunno if err(3) counts as part of a sane libc
or if it's worth spending time on such a patch?

-- 
Tony Finch  <dot at dotat.at>  https://dotat.at/
Fair Isle, Faeroes: Variable 2 to 4 in west Faeroes at first,
otherwise southeasterly 4 to 6. Moderate, occasionally rough at first
in east Fair Isle. Showers, fog patches later. Good, occasionally very
poor later.