FRAG / FRAG2: CONFIG event overrides user-configured frag_size value

When we have multiple FRAG or FRAG2 in the same stack (with different IDs), then each fragmentation protocol sends a CONFIG event (with frag_size) up and down the stack. When this event is received by a different frag protocol, then it will set its own frag_size, overriding the previously set value.
Investigate whether we still need the CONFIG event.