std.posix: recvmsg

[cursedquail]

Lifted from #19751 verbatim - thanks to @truemedian for the implementation 😅. That PR has been closed, and even though the larger effort around io has been subsumed by Andrew's writer refactoring, I didn't see this in the wrangle-writer-buffering branch. I decided it was probably ok to resurrect this bit of code.

I did spot-check the errors against the man pages and made sure all the listed errors in that page are present here - although I'll admit I don't truly grasp why some errors are marked as unreachable? I suppose we assume that this function will be called with proper arguments, and any issues around that will be caught in debug builds, where those unreachables will be printed with a stacktrace

Closes #20660

[dec05eba]

To completely close #20660 it should also be possible to send and receive file descriptors, which is done in C with CMSG macro and cmsghdr struct, which appears to have different definitions on different architectures/os.
This is what I have to do in my zig program that uses recvmsg with file descriptors to make it work on x86_64 linux:

const SCM_RIGHTS: i32 = 1;

const cmsghdr = extern struct {
    len: usize, // TODO: This size is different on different OS'
    level: i32,
    type: i32,
};

inline fn cmsg_align(size: usize) usize {
    return std.mem.alignForward(usize, size, @sizeOf(usize));
}

inline fn cmsg_space(size: usize) usize {
    return cmsg_align(@sizeOf(cmsghdr)) + cmsg_align(size);
}

inline fn cmsg_len(size: usize) usize {
    return cmsg_align(@sizeOf(cmsghdr)) + size;
}

and then to set the data in the code:

var cmsgbuf: [cmsg_space(@sizeOf(std.posix.fd_t) * max_fds)]u8 align(@alignOf(cmsghdr)) = undefined;
var cmsg: *cmsghdr = @ptrCast(&cmsgbuf);
cmsg.level = std.posix.SOL.SOCKET;
cmsg.type = SCM_RIGHTS;
cmsg.len = @intCast(cmsg_len(@sizeOf(std.posix.fd_t) * fds_to_send.len));

var fds = std.mem.bytesAsSlice(std.posix.fd_t, cmsgbuf[@sizeOf(cmsghdr)..]);
@memcpy(fds[0..fds_to_send.len], fds_to_send);

it would be nice if zig could provide a nicer interface to this in std.posix. But I guess this should be done in a separate commit?