RFC 5837 - 4.3. Interface Name Sub-Object
"The Interface Name Sub-Object MUST have a length that is a multiple
of 4 octets and MUST NOT exceed 64 octets.
The Length field represents the length of the Interface Name Sub-
Object, including the length and the interface name in octets."
The length of the interface name to print is: (Length field) - 1.
The offset is only: Length field
Add sanity checks: Multiple of 4 octets, <= 64 octets.
Fix the icmp-rfc5837.pcap test.
The interface name length in octets is 63, thus the length field must be 64.
Update also the ICMP Multi-Part Extensions checksum.
Keep an invalid length field in icmp-cksum-oobr-2.pcap.
Update the output of two tests accordingly.
