src: make interval sets work with string datatypes

src: make interval sets work with string datatypes

Allows to interface names in interval sets:

table inet filter {
        set s {
                type ifname
                flags interval
                elements = { eth*, foo }
        }

Concatenations are not yet supported, also, listing is broken,
those strings will not be printed back because the values will remain
in big-endian order.  Followup patch will extend segtree to translate
this back to host byte order.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>