NFD

Currently, NFD checks if the unix socket file exists and if so simply deletes the file. This causes problems when NFD is run twice (i.e., the second attempt will fail because it will not be able to bind to tcp/udp socket, but will disable any future communication via unix socket).

This task is to change this behavior. Instead of just deleting existing file, the initialization procedure should:

1. if UNIX socket doesn’t exist, goto step 4
2. connect to the UNIX socket, if connection succeeds (indicating another process is listening on the same socket), raise an error and abort these steps
3. delete the UNIX socket
4. start listening