std.os.setsockopt broken with libc

[dermetfan]

std.os.setsockopt appears to be broken when compiling with libc.

Example:

usingnamespace @import("std");

pub fn main() !void {
    const fd = try os.socket(os.AF_INET, os.SOCK_STREAM, 0);
    _ = try os.setsockopt(fd, os.SOL_SOCKET, os.SO_REUSEADDR, &mem.toBytes(@as(c_int, 1)));

    // The same bug breaks StreamServer:

    var server = net.StreamServer.init(.{});
    try server.listen(try net.Address.parseIp4("127.0.0.1", 12345));
}

This compiles fine with zig build-exe main.zig but gives the following error when compiling with --library c:

/nix/store/syj90d418f63szyinbklp69zvv10v8qy-zig-5acc8afb5f9674b9b7b290635e9c2837872b8a93/lib/zig/std/os.zig:4228:60: error: cast discards const qualifier
    switch (errno(system.setsockopt(fd, level, optname, opt.ptr, @intCast(socklen_t, opt.len)))) {
                                                           ^

I got around this issue by removing the const qualifier in my copy of the standard library:

- pub fn setsockopt(fd: fd_t, level: u32, optname: u32, opt: []const u8) SetSockOptError!void {
+ pub fn setsockopt(fd: fd_t, level: u32, optname: u32, opt: []u8) SetSockOptError!void {

I'm not sure if that's an ok fix though so I did not open a PR.