When file capabilities are granted with `sudo setcap ...+p`, capabilities
are placed into the thread's permitted set rather than its effective set.
While TUN interface provisioning in `tsunagi` properly acquired and dropped
CAP_NET_ADMIN using `caps::raise`, `tsunagi-tcp-tls` attempted to bind port 443
directly without activating CAP_NET_BIND_SERVICE, resulting in EACCES
(os error 13 Permission denied).
Fixes and improvements:
- Add `caps` dependency to `tsunagi-tcp-tls` on Linux.
- Implement `NetBindServiceGuard` to temporarily raise CAP_NET_BIND_SERVICE
into the effective set for the synchronous `TcpListener::bind` invocation
and immediately drop it afterwards.
- Clarify diagnostic warnings to distinguish between default port 443 fallback
and explicitly requested port failure.
- Update all privilege diagnostics and error hints across the codebase
to uniformly recommend granting both capabilities:
`sudo setcap cap_net_admin,cap_net_bind_service+p <path>`.
Validation on Linux:
- `cargo fmt --all -- --check` passed.
- `cargo clippy --locked --workspace --all-targets -- -D warnings` passed.
- `cargo test --locked --workspace --all-targets` passed (all workspace unit,
integration, and protocol tests passed).
Aleksandr Bogomiakov
5495cb5b34