If "-n" is specified along with "-e", print the address family value as

If "-n" is specified along with "-e", print the address family value as
"AF <n>" for DLT_NULL captures.

Print the packet length for DLT_NULL captures when "-e" is specified
(somebody complained on "freebsd-isp" about the lack of any packet
length from tcpdump when capturing traffic on a "tunX" device).

Make sure we always print the packet length on PPP when "-e" is
specified (the person in question also complained that the packet length
disappeared between FreeBSD 2.2.8 and FreeBSD 3.4, which was the result
of a new "print-ppp.c" that didn't print the length).