Update seccomp patch, based on staging patch

[gasinvein]

Adopted patch from ValveSoftware/wine#99 (comment)

Original Wine code from Proton (as before #5)

    int ret;

    if (!(ret = prctl(PR_GET_SECCOMP, 0, NULL, 0, 0)))
    {
        if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
        {
            perror("prctl(PR_SET_NO_NEW_PRIVS, ...)");
            exit(1);
        }

        if (sc_seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog))

        {
            perror("prctl(PR_SET_SECCOMP, ...)");
            exit(1);
        }

        check_bpf_jit_enable();
    }
    else
    {
        if (ret == 2)
            TRACE("Seccomp filters already installed.\n");
        else
            ERR("Seccomp filters cannot be installed, ret %d, error %s.\n", ret, strerror(errno));
    }

    sigaction(SIGSYS, sig_act, NULL);

Patched code (after this PR)

    NTSTATUS status;

    sigaction(SIGSYS, sig_act, NULL);

    if ((status = syscall(0xffff)) == STATUS_INVALID_PARAMETER)
    {
        TRACE("Seccomp filters already installed.\n");
        return;
    }
    if (status != -ENOSYS && (status != -1 || errno != ENOSYS))
    {
        ERR("Unexpected status %#x, errno %d.\n", status, errno);
        return;
    }


    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
    {
        perror("prctl(PR_SET_NO_NEW_PRIVS, ...)");
        exit(1);
    }
    if (sc_seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog))
    {
        perror("prctl(PR_SET_SECCOMP, ...)");
        exit(1);
    }
    check_bpf_jit_enable();

Since I'm not familiar with C and not sure if got everything right, a review from someone more knowledgeable would be very useful.

[flathubbot]

Started test build 31985

[flathubbot]

Build 31985 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/30801/com.valvesoftware.Steam.CompatibilityTool.Proton.flatpakref

[nanonyme]

LGTM. If hasn't gone through battle testing yet, maybe let's get it tested by end-users while n beta.

[gasinvein]

Ok, let's merge this to beta. Review is still wanted before it lands to master.