Do not destroy PortForwarding if other applications using same namespace

[jamesmcm]

Noticed with AzireVPN:

 2025-08-26T20:40:59.249Z INFO  vopono::exec                                    > Application firefox-developer-edition launched in network namespace vo_az_sweden with pid 221146
 2025-08-26T20:40:59.249Z INFO  vopono::exec                                    > Port Forwarding on port 58912
[GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt
 2025-08-27T07:23:36.943Z INFO  vopono_core::network::port_forwarding::azirevpn > AzireVPN Port forwarding destroyed: {
    "status": "success",
    "data": []
}
 2025-08-27T07:23:36.964Z DEBUG vopono_core::util                               > Using config dir from $HOME config: /home/archie/.config
 2025-08-27T07:23:36.964Z DEBUG vopono_core::util                               > Using config dir from $HOME config: /home/archie/.config
 2025-08-27T07:23:36.964Z DEBUG vopono_core::network::netns                     > Skipping destructors since other vopono instance using this namespace!
 2025-08-27T07:23:36.964Z DEBUG vopono_core::network::netns                     > Existing lockfiles using this namespace: [Ok(DirEntry("/home/archie/.config/vopono/locks/vo_az_sweden/222063"))]

[jamesmcm]

Unfortunately due to the need to hand-over Port Forwarding renewal for piapf and natpmpc - this is very complicated to deal with using just Lockfiles (especially with the current RAII set up) - this should be better addressed in #298 where the central daemon could take over the renewal.