Add a bunch of bounds checks.

Add a bunch of bounds checks.

Don't bother copying the route header - we're using the EXTRACT_ macros,
so we don't have to worry about alignment.

Use the length field from the packet, not just the length of the
containing packet, and check to make sure the containing packet is long
enough to contain it.

Add a bunch of checks against the packet length.

In "default_print()" calls, cut the length off at the captured data
length.  In other calls, don't - let the bounds checks handle caplen <
length.