[RFC] unixmon virtual device for unix sockets

There is currently no way to capture a connected unix domain socket
without interrupting the connection. AF_UNIX socket transport is
implemented in-kernel and enabling packet captures would require
kernel support. This could be based on the design of the vsockmon and
nlmon virtual devices.

This would be very useful when debugging traffic traversing AF_UNIX
sockets, such as DBUS traffic without modifying the user-space
programs that are using the unix sockets.

Is this a worthwhile feature to implement?

-Josh