From Mark C. Brown: add support for FDDI and Token Ring cards.
Set the file encapsulation the same way it's done for iptrace captures -
leave it as "unknown" to start with, and, for each packet we see, set it
to the packet's encapsulation type if the file encapsulation type is
unknown and set it to "per-packet" if the file encapsulation type is
"known" but isn't the type of that packet, so files that have all the
same type of packet have that type as the file type and packets that
*don't* have all the same type of packet have "per-packet".
svn path=/trunk/; revision=10543