std.c.NI was never used in the source, so let's finally use it and make
the function more clear!

This is a breaking change, although a minor one: If you previously passed 0 here
(meaning no flags), then now you have to pass an empty struct (.{}) instead.
Otherwise, you probably used @bitCast() shenanigans here (like
@bitCast(c.NI { .NUMERICHOST = true }) and that will still work, but you can
also get rid of the @bitCast() now!