Enough LXC to create separate networking stack and to bring up a usable getty.

lxc2.c

#include <sched.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int child(void *n) {
    char buf[512] = {0};
    mount("/proc", "/proc", "proc", 0, NULL);
    // mount("devpts", "/dev/pts", "devpts", 0, "newinstance,ptmxmode=0666,mode=0620,gid=5");
    system("ifconfig slave 192.168.0.190/24 up");
    close(2);
    close(1);
    close(0);
    sprintf(buf, "agetty -L 38400 pts/%d vt100", *((int *)n));
    system(buf);
    sleep(30);
    return 0;
}

void main(void) {
    int cflags;
    int stacksize = getpagesize() * 4;
    char *stack, *stacktop;
    char buf[1024] = {0}; // Bless me Father, for I have sinned.
    int ptmx;
    int unlock = 0;
    int ptyno;
    pid_t pid;
    void *pointer;
    int val;

    cflags = CLONE_NEWPID|CLONE_NEWNS|CLONE_NEWUTS|CLONE_NEWIPC|SIGCHLD|CLONE_NEWNET;

    stack = (char *)malloc(stacksize);
    stacktop = stack + stacksize;

    system("ip link add name master type veth peer name slave");
    system("ifconfig master up");
    system("brctl addif br1 master");
    ptmx = open("/dev/pts/ptmx", O_RDWR|O_NOCTTY|O_NONBLOCK);
    ioctl(ptmx, TIOCSPTLCK, &unlock);
    pointer = (void *)malloc(1);
    ioctl(ptmx, TIOCGPTN, pointer);
    sleep(5); // Give the bridge a chance to spin up
    pid = clone(child, stacktop, cflags, pointer);
    sprintf(buf, "ip link set slave netns %d", pid);
    system(buf);
    val = fcntl(0, F_GETFL, 0);
    fcntl(0, F_SETFL, val|O_NONBLOCK);

    for(;;) {
        int i = 0;
        i = read(ptmx, buf, 512);
        if(i > 0) {
            write(1, buf, i);
        } else {
            sleep(1);
        }
        i = read(0, buf, 512);
        if(i > 0) {
            write(ptmx, buf, i);
        }
    }
}

That's an unforgivably bad read-write loop at the bottom of main, but it's late and I've gotta go to bed. select() is for people with time on their hands.