Clean up IS-IS printing.

Clean up IS-IS printing.

Check the packet length as well as the captured length.

Don't have separate "non-verbose" and "verbose" cases, so we don't
duplicate the length checks.

Print the right value for some length check errors.

Clean up the TLV parsing code - keep parsing until we completely run out
of data, but make sure, before we look at the TLV header, that we
haven't run past the end of the packet as well as that we haven't run
past the end of the captured data.

In the OSI checksum routine:

Have it take an unsigned length, so we don't have to worry about it
being negative.

Use ND_TTEST2 to check whether the checksum offset is within the
bounds of the packet.

If we don't have all the packet data to checksum, just report
"unverified", don't return a "truncated" error.