1) Set interface up/down correctly as a function of open and close of the    SLIP device. 2) Don't directly frob the IFF_UP flag - use if_up/if_down as it was    intended. 3) Return ENETDOWN if IFF_UP isn't set when outputing, drop the packet if    if IFF_UP isn't set when inputing.
1) Set interface up/down correctly as a function of open and close of the
   SLIP device.
2) Don't directly frob the IFF_UP flag - use if_up/if_down as it was
   intended.
3) Return ENETDOWN if IFF_UP isn't set when outputing, drop the packet if
   if IFF_UP isn't set when inputing.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase