More EXTRACT_8BITS() and other cleanups.

Get rid of casts to (int) that aren't needed or wanted.

If a field is unsigned, use an unsigned variable for it, print it with
%u, not %d, and don't cast it to int.

Replace a static variable in print-dvmrp.c with a local variable in
dvmrp_print() and a parameter to print_neighbors2().