Correct handling of closed TCP connections in proto_inet

Correct handling of closed TCP connections in proto_inet

In FRSH_FORB this allows restarting resource managers without restart of fcb.
The problem of previous implementation was, that if the socket was closed but
peer structure was not unregistered the disconnected peer structure stored
fd of closed socket and when a new connection was made, the fd was the same
as stored in disconnected peer structure. Therefore the disconnected peer
appeared as alive.