outgoing_interfaces: silent fallback to default route when bind_device fails

[PoSFFS]

When outgoing_interfaces is set and bind_device() fails, session_impl::bind_outgoing_socket() logs the error but does not abort. It continues to call connect() on the now-unbound socket, causing traffic to silently go out on whatever interface the OS routing table prefers.

Impact:
On machines with multiple network interfaces, this is particularly problematic — traffic intended for one network will silently leak onto another with no indication to the user.

Location:
src/session_impl.cpp, bind_outgoing_socket() — bind_device() failure does not prevent connect() from being called.

Requested Solution:
Close the socket and return on bind_device() failure. Do not connect.

if (!device.empty()) {
    bind_device(s, device.c_str(), ec);
    if (ec) {
        s.close();
        return;
    }
}

Suggestion:
Additionally, fire peer_blocked_alert with invalid_local_interface so the caller can diagnose the misconfiguration.

if (m_alerts.should_post<peer_blocked_alert>())
    m_alerts.emplace_alert<peer_blocked_alert>(
        torrent_handle(),
        endpoint,
        peer_blocked_alert::invalid_local_interface);

Reason:
If a user specifies X and X fails, Y should not be used silently. I make this request because i do not want to solve a internal issue with a external program. Even thou my stance is to always use a firewall and a proper routing table.

Thank you for maintaining libtorrent — I have great appreciation for your work.