mergesort: find base value expression type via recursion

mergesort: find base value expression type via recursion

Sets that store flags might contain a mixture of values and binary
operations. Find the base value type via recursion to compare the
expressions.

Make sure concatenations are listed in a deterministic way via
concat_expr_msort_value() which builds a mpz value with the tuple.

Adjust a few tests after this update since listing differs after this
update.

Fixes: 14ee0a979b62 ("src: sort set elements in netlink_get_setelems()")
Fixes: 3926a3369bb5 ("mergesort: unbreak listing with binops")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>