add osmo_fsm_set_dealloc_ctx(), to help with use-after-free

This is a simpler and more general solution to the problem so far solved by
osmo_fsm_term_safely(true). This extends use-after-free fixes to arbitrary
functions, not only FSM instances during termination.

The aim is to defer talloc_free() until back in the main loop.

Rationale: I discovered an osmo-msc use-after-free crash from an invalid
message, caused by this pattern:

void event_action()
{
       osmo_fsm_inst_dispatch(foo, FOO_EVENT, NULL);
       osmo_fsm_inst_dispatch(bar, BAR_EVENT, NULL);
}

Usually, FOO_EVENT takes successful action, and afterwards we also notify bar.
However, in this particular case, FOO_EVENT caused failure, and the immediate
error handling directly terminated and deallocated bar. In such a case,
dispatching BAR_EVENT causes a use-after-free; this constituted a DoS vector
just from sending messages that cause *any* failure during the first event
dispatch.

Instead, when this is enabled, we do not deallocate 'foo' until event_action()
has returned back to the main loop.

Test: duplicate fsm_dealloc_test.c using this, and print the number of items
deallocated in each test loop, to ensure the feature works. We also verify that
the deallocation safety works simply by fsm_dealloc_test.c not crashing.

We should probably follow up by refusing event dispatch and state transitions
for FSM instances that are terminating or already terminated:
see I0adc13a1a998e953b6c850efa2761350dd07e03a.

Change-Id: Ief4dba9ea587c9b4aea69993e965fbb20fb80e78

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