Apply a seccomp-BPF syscall allowlist by default

Derived from Podman's default profile, stripped of capability-conditional
rules (we never grant capabilities), argument filters, and the explicit
EPERM block. Dangerous syscalls (mount, unshare, ptrace, bpf,
perf_event_open, io_uring_*, keyctl, kexec_*, ...) fall through to the
default ENOSYS action, which also keeps glibc's clone3 -> clone fallback
working. x86_64 and aarch64 are supported; other archs error out.

Toggle with --seccomp / --no-seccomp or seccomp = <bool> in config.

Cargo.toml

@@ -14,8 +14,11 @@ path = "src/main.rs"
 [dependencies]
 clap = { version = "4", features = ["derive"] }
 glob = "0.3"
+libc = "0.2"
+seccompiler = "0.5"
 serde = { version = "1", features = ["derive"] }
 shlex = "1.3.0"
+syscalls = { version = "0.8", default-features = false, features = ["std"] }
 toml = "1"
 
 [dev-dependencies]