OptsVisitor: introduce list modes for interval flattening

The new modes are equal-rank, exclusive alternatives of LM_IN_PROGRESS.
Teach opts_next_list(), opts_type_int() and opts_type_uint64() to handle
them.

Also enumerate explicitly what functions are valid to call in what modes:
- opts_next_list() is valid to call while flattening a range,
- opts_end_list(): ditto,
- lookup_scalar() is invalid to call during flattening; generated qapi
  traversal code must continue asking for the same kind of signed/unsigned
  list element until the interval is fully flattened,
- processed(): ditto.

List mode restrictions are always formulated in positive / inclusive
sense. The restrictions for lookup_scalar() and processed() are
automatically satisfied by current qapi traversals if the schema to build
is compatible with OptsVisitor.

The new list modes are not entered yet.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
