The following assertions fail on non-Linux platforms after c0c20105354
which inserted padding based on musl definitions. This padding only
exists on musl to workaround a discrepancy betweeen the POSIX API and
Linux ABI, and is incorrect on other POSIX operating systems.

This change makes the padding musl-only, and documents the reason it
exists. With this change, the assertions pass on Linux and FreeBSD
targets. The corresponding definitions on other targets line up with the
POSIX and FreeBSD ones, so they should work there too.

```zig
const std = @import("std");
const assert = std.debug.assert;
const msghdr = std.c.msghdr;
const cmsghdr = std.c.cmsghdr;

const c = @cImport({
    @cInclude("sys/socket.h");
});

comptime {
    assert(@offsetOf(msghdr, "iovlen") == @offsetOf(c.msghdr, "msg_iovlen"));
    assert(@offsetOf(msghdr, "controllen") == @offsetOf(c.msghdr, "msg_controllen"));
    assert(@offsetOf(msghdr, "control") == @offsetOf(c.msghdr, "msg_control"));
    assert(@offsetOf(msghdr, "flags") == @offsetOf(c.msghdr, "msg_flags"));

    assert(@sizeOf(msghdr) == @sizeOf(c.msghdr));

    assert(@offsetOf(cmsghdr, "len") == @offsetOf(c.cmsghdr, "cmsg_len"));
    assert(@offsetOf(cmsghdr, "level") == @offsetOf(c.cmsghdr, "cmsg_level"));

    assert(@sizeOf(cmsghdr) == @sizeOf(c.cmsghdr));
}
```