The intent was to run custom code after the user is done modifying the socket,
for example to update a (not yet existing) port->socket map in SocketSet. However
this wouldn't work, since the SocketRef would have to borrow the SocketSet at
the same time as the Socket to be able to notify the SocketSet.
I believe such indexing can be achieved by setting a "dirty" bit *before* giving
the socket to the user, then on poll() reindexing all dirty sockets. This could
even be faster: if user gets a socket multiple times between polls, it'd be reindexed
only once.
Also check for the correct destination PAN id when receiving a frame (as
discussed). Linux does this as well.
However, hardware implementations also can drop those packets.
Using a raw socket on `monitor0` causes weird results: packets we receive
include FCS, packets we send are parsed as if they didn't have FCS, except
by wireshark which always expects a FCS??
Turns out the sane way is to use raw sockets on normal `wpanX` interfaces,
in which case all packets we send/receive are without FCS.
