void do_unshare(void)
{
	FILE *f;
	uid_t uid = geteuid();
	gid_t gid = getegid();
	unshare(CLONE_NEWNS|CLONE_NEWUSER);
	f = fopen("/proc/self/uid_map", "w");
	fprintf(f, "0 %d 1", uid);
	fclose(f);
	f = fopen("/proc/self/setgroups", "w");
	fprintf(f, "deny");
	fclose(f);
	f = fopen("/proc/self/gid_map", "w");
	fprintf(f, "0 %d 1", gid);
	fclose(f);
	mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL);
}

This obviously needs error checking - in this form it won't do
anything good without userns enabled (coredump on the first
fprintf() in there, since there won't be /proc/self/uid_map);
should probably just report CLONE_NEWUSER failure, warn about
skipped tests, fall back to unshare(CLONE_NEWNS) and skip
everything in in_child()...