src: detach set, list and concatenation expression layout

These three expressions use the same layout, but they have a different
purpose. Several fields are specific of a given expression:

- set_flags is only required by set expressions.
- field_len and field_count are only used by concatenation expressions.

Add accessors to validate the expression type before accessing the union
fields:

 #define expr_set(__expr)       (assert((__expr)->etype == EXPR_SET), &(__expr)->expr_set)
 #define expr_concat(__expr)    (assert((__expr)->etype == EXPR_CONCAT), &(__expr)->expr_concat)
 #define expr_list(__expr)      (assert((__expr)->etype == EXPR_LIST), &(__expr)->expr_list)

This should help catch subtle bugs due to type confusion.

assert() could be later enabled only in debugging builds to run tests,
keep it by now.

compound_expr_*() still works and it needs the same initial layout for
all of these expressions:

      struct list_head        expressions;
      unsigned int            size;

This is implicitly reducing the size of one of the largest structs
in the union area of struct expr, still EXPR_SET_ELEM remains the
largest so no gain is achieved in this iteration.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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