Spring cleanup

Spring cleanup

- pointless casts, kill caddr_t or replace it with char *
- signed counters
- simplify if_getmtu, only one method is needed and SIOCGIFMTU is the
  cheapest
- we no longer have drivers for IFT_FDDI
- hide details of iflist management
- if (dflag) log_debug -> log_debug
- dead code and comments
- etc etc

Input from and ok florian@