[supervisor] Show process that serves port

At the moment we scan /proc/net/tcp{6} to detect which ports are being used/served in a workspace. While this works reliably, it has the major downside that we cannot tell wich process serves this port - information that would be handy to display. Also, because of the polling delay, there's a delay in port detection.

Possible solutions

seccomp-notify

User-namespaced workspaces make use of seccomp-notify. We could get notifications on accept, and use the sockfd to get more information about the socket (which port, which address). workspacekit would then provide a Unix socket on which we serve port status information (either re-using supervisor's StatusService or coming up with our own).

Alternatively, workspacekit could provide a file in a well-known location that offers port -> PID mappings, which we'd use to enhance the port information.

scanning /proc

To identify the process that's serving a port, we could also scan /proc and match the sockets inode with the process FD. With proper caching it's conceivable that this solution could come in at an acceptable CPU/performance cost.

Start by tracing supervisor's current /proc/net/tcp{6} port scan and reviewing components/supervisor-api/status.proto, then compare the seccomp-notify and /proc approaches described in the issue. Done means port status includes the serving process and avoids the current detection delay, but the implementation direction must first be settled.