Error out if the interface does not come up

Wait for 2 seconds for the interface to become ready. On my netbook,
getting the datalink type will sometimes loop forever. When I ran tcpdump
on the interface there weren't any frames received. The interface seems
to be stuck.

Bringing the interface up and down, etc, didn't help but rebooting did.

Had something to do with the Ubuntu network manager running at the same
time the interface was being toggled into monitor mode. Usually this
works ok but it's better to shut down the network manager first:

service network-manager stop

src/wierl_monitor.erl

@@ -249,12 +249,19 @@ dlt(wierl_radiotap) -> 803.
 % from the socket. The interface may not be ready, so spin
 % here until it comes up.
 %
-% XXX May end up looping forever here.
+% In some cases the interface will never come up. tcpdump
+% doesn't display any frames being received by the interface.
+% Only fix so far is to reboot.
 datalinktype(Socket) ->
+    datalinktype(Socket, 200).
+
+datalinktype(_Socket, 0) ->
+    throw({error,enetdown});
+datalinktype(Socket, N) ->
     case read(Socket, 0) of
         {error,eagain} ->
             timer:sleep(10),
-            datalinktype(Socket);
+            datalinktype(Socket, N-1);
         {ok, <<>>} ->
             ok
     end.