ICMP extension structures allow a few existing ICMP error messages to
convey extra information for troubleshooting; especially the root cause
of why the original datagram could not be delivered.

This CL adds generic support for ICMP extension stuructures to
DstUnreach, TimeExceeded and ParamProb structs. Specific extensions such
as MPLS label-stack, interface and next-hop identification will be
inplemneted in separate followup CLs.

Change-Id: I90798c135bdf76b806e2dde2bdd57c2c11d7e7e9