src: add auto-dependencies for ipv4 icmp

src: add auto-dependencies for ipv4 icmp

The ICMP header has field values that are only exist
for certain types.

Mark the icmp proto 'type' field as a nextheader field
and add a new th description to store the icmp type
dependency.  This can later be re-used for other protocol
dependend definitions such as mptcp options -- which are all share the
same tcp option number and have a special 4 bit marker inside the
mptcp option space that tells how the remaining option looks like.

Signed-off-by: Florian Westphal <fw@strlen.de>