segtree: add string "range" reversal support

segtree: add string "range" reversal support

Previous commits allows to use set key as a range, i.e.

	key ifname
	flags interval
	elements = { eth* }

and then have it match on any interface starting with 'eth'.

Listing is broken however, we need to reverse-translate the (128bit)
number back to a string.

'eth*' is stored as interval
00687465 0000000 ..  00697465 0000000, i.e. "eth-eti",
this adds the needed endianess fixups.

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