optimize: expand implicit set element when merging into concatenation

optimize: expand implicit set element when merging into concatenation

Generalize the existing code to deal with implicit sets. When merging a
ruleset like the following:

	udp dport 128 iifname "foo"		#1
        udp dport { 67, 123 } iifname "bar"	#2

into a concatenation of statements, the following expansion need to
be done for rule #2:

	67 . "bar"
	123 . "bar"

The expansion logic consists of cloning the existing concatenation being
built and then append each element in the implicit set. A list of
ongoing concatenations being built is maintained, so further expansions
are also supported.

Extend test to cover for this use-case.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1628
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>