Support IP_MULTICAST_IF socket option in linux-user
Additional information:
I've run into this limitation in qemu-aarch64-static version Debian 1:6.2+dfsg-2ubuntu6.12, but from the link above, it doesn't seem to be implemented on master yet.

Here's some source code that demonstrates the failure:
```
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/ip.h>
#include <unistd.h>
#include <assert.h>
#include <stdio.h>

int main()
{
    int fd, ret;
    struct in_addr addr = {htonl(INADDR_LOOPBACK)};

    fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    assert(fd >= 0);
    ret = setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
    if (ret < 0)
    {
        perror("setsockopt failed");
        return 1;
    }
    close(fd);
    printf("Success!\n");
    return 0;
}
```

When run under qemu, it gives the error `setsockopt failed: Protocol not available`.

It doesn't look like it should be too hard to support (certainly no worse than IP_ADD_MEMBERSHIP). Let me know if I can help with a patch.