src: infer NAT mapping with concatenation from set

src: infer NAT mapping with concatenation from set

If the map is anonymous, infer it from the set elements. Otherwise, the
set definition already have an explicit concatenation definition in the
data side of the mapping.

This update simplifies the NAT mapping syntax with concatenations, e.g.

 snat ip to ip saddr map { 10.141.11.4 : 192.168.2.3 . 80 }

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