ipc, conn: resource-leak close file descriptors on error paths

[fereidani]

Hi,

I've built a static analyzer for Go called Ghoul, and I ran it against the wireguard-go codebase. It flagged a few error paths where a file descriptor is opened but never closed.

Ghoul reports:

ipc/uapi_linux.go:77:3: [resource-leak] resource from net.FileListener allocated at ipc/uapi_linux.go:56 never closed (confidence: 6.4/10, CWE-404)
ipc/uapi_linux.go:89:3: [resource-leak] resource from net.FileListener allocated at ipc/uapi_linux.go:56 never closed (confidence: 6.4/10, CWE-404)
ipc/uapi_linux.go:95:3: [resource-leak] resource from net.FileListener allocated at ipc/uapi_linux.go:56 never closed (confidence: 6.4/10, CWE-404)
ipc/uapi_unix.go:56:3: [resource-leak] resource from net.Dial allocated at ipc/uapi_unix.go:55 never closed (confidence: 6.4/10, CWE-404)

This series closes each leaked descriptor on its error path:

• ipc/uapi_unix.go: close the unix connection opened for the socket liveness probe, and close the listener when duplicating its file descriptor fails.
• ipc/uapi_linux.go: close the listener if inotify initialization, watch addition, or the rwcancel helper fails; close the inotify file descriptor as well when watch addition fails.
• conn/bind_std.go: close the listening socket in listenNet when resolving its local address fails.

Each fix is focused on a single path and lives in its own commit.