MINOR: spoe: Improve implementation of the payload fragmentation

MINOR: spoe: Improve implementation of the payload fragmentation

Now, when a payload is fragmented, the first frame must define the frame type
and the followings must use the special type SPOE_FRM_T_UNSET. This way, it is
easy to know if a fragment is the first one or not. Of course, all frames must
still share the same stream-id and frame-id.

Update SPOA example accordingly.