`set_secure_computing_mode(SecureComputingMode::Filter)` has no filter argument

The C interface looks like

```C
[[deprecated]]
int prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, struct sock_fprog *filter);
```

however rustix' iterface looks like

```Rust
set_secure_computing_mode(SecureComputingMode::Filter)
```

so `filter` will be some garbage, mostly likely failing with `EFAULT`.

Since `PR_SET_SECCOMP` (but not `PR_GET_SECCOMP`) is deprecated and superseded by `seccomp` anyway. Do you see a good reason to fix the prctl variant? Or should we just deprecated and later remove the prctl variant in rustix and add support for `seccomp(2)` as `seccomp_set_mode_strict()` and `seccomp_set_mode_filter(flags, args)`?