marshal: fix list marshalling and various other changes

marshal: fix list marshalling and various other changes

Since last element and previous element are a pointer to a pointer, we
should handle this case differently. Unit tests are not failing
because the original memory space is still available on the stack. We
just ignore last and previous. This still allows to use the list for
the most useful operations: TAILQ_EMPTY, TAILQ_FOREACH and
TAILQ_REMOVE. We can't use TAILQ_INSERT_TAIL!

Another important change is that by default, macros will declare an
external structure. MARSHAL_EXPORT variable should be defined only for
one module. Otherwise, marshal structures will be defined several
times.

We also rename `_marshal_*` to `marshal_*_`. This enables more
graceful logging messages.