evaluate: set: Allow for set elems to be sets

evaluate: set: Allow for set elems to be sets

Recursive use of sets is handled in parts by parser_bison.y, which
has a rule for inline unnamed sets in set_list_member_expr, e.g. like
this:

| add rule ip saddr { { 1.1.1.0, 2.2.2.0 }, 3.3.3.0 }

Yet there is another way to have an unnamed set inline, which is via
define:

| define myset = {
| 	1.1.1.0,
| 	2.2.2.0,
| }
| add rule ip saddr { $myset, 3.3.3.0 }

This didn't work because the inline set comes in as EXPR_SET_ELEM with
EXPR_SET as key. This patch handles that case by replacing the former by
a copy of the latter, so the following set list merging can take place.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>