std.c: Use std.c.NI as the flags type in getnameinfo

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!

lib/std/c.zig

@@ -9774,7 +9774,7 @@ pub extern "c" fn getnameinfo(
     hostlen: socklen_t,
     noalias serv: ?[*]u8,
     servlen: socklen_t,
-    flags: u32,
+    flags: NI,
 ) EAI;
 
 pub extern "c" fn gai_strerror(errcode: EAI) [*:0]const u8;