src: ct: store proto base of ct key, if any

src: ct: store proto base of ct key, if any

ct keys can match on network and tranasport header protocol
elements, such as port numbers or ip addresses.

Store this base type so a followup commit can store and kill
dependencies, e.g. if bsae is network header we might be able
to kill an earlier expression because the dependency is implicit.

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