src: sort set elements in netlink_get_setelems()

src: sort set elements in netlink_get_setelems()

So users can better track their ruleset via git.

Without sorting, the elements can be listed in a different order
every time the set is created, generating unnecessary git changes.

Mergesort is used. Doesn't sort sets with 'flags interval' set on.

Pablo appends to this changelog description:

Currently these interval set elements are dumped in order. We'll likely
get new representations soon that may not guarantee this anymore, so
let's revisit this later in case we need it.

Without this patch, nft list ruleset with a set containing 40000
elements takes on my laptop:

	real    0m2.742s
	user    0m0.112s
	sys     0m0.280s

With this patch:

	real    0m2.846s
	user    0m0.180s
	sys     0m0.284s

Difference is small, so don't get nft more complicated with yet another
getopt() option, enable this by default.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>