src: allow to specify the base chain type

src: allow to specify the base chain type

This patch allows you to specify the type of the base chain, eg.

	add table mangle
	add chain mangle OUTPUT { type route hook NF_INET_LOCAL_OUT 0; }

The chain type determines the semantics of the chain, we currently
have three types:

* filter, used for plain packet filtering.
* nat, it only sees the first packet of the flow.
* route, which is the equivalent of the iptables mangle table, that
  triggers a re-route if there is any change in some of the packet header
  fields, eg. IP TOS/DSCP, or the packet metainformation, eg. mark.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>