Platform invokes change events while reading netlink events. However,
platform code is not re-entrant and calling into platform again is not
allowed (aside operations that do not process the netlink socket, like
lookup of the platform cache).

That basically means, we have to always process events in an idle
handler. That is not a too strong limitation, because we anyway don't
know the call context in which the platform event is emitted and we
should avoid unguarded recursive calls into platform.

Otherwise, we get hit an assertion/crash in nm-iface-helper:

     1  raise()
     2  abort()
     3  g_assertion_message()
     4  g_assertion_message_expr()
     5  do_delete_object()
     6  ip6_address_delete()
 >>> 7  nm_platform_ip6_address_delete()
     8  nm_platform_ip6_address_sync()
     9  nm_ip6_config_commit()
     10 ndisc_config_changed()
     11 ffi_call_unix64()
     12 ffi_call()
     13 g_cclosure_marshal_generic_va()
     14 _g_closure_invoke_va()
     15 g_signal_emit_valist()
     16 g_signal_emit()
 >>> 17 nm_ndisc_dad_failed()
     18 ffi_call_unix64()
     19 ffi_call()
     20 g_cclosure_marshal_generic()
     21 g_closure_invoke()
     22 signal_emit_unlocked_R()
     23 g_signal_emit_valist()
     24 g_signal_emit()
 >>> 25 nm_platform_cache_update_emit_signal()
     26 event_handler_recvmsgs()
     27 event_handler_read_netlink()
     28 delayed_action_handle_one()
     29 delayed_action_handle_all()
     30 do_delete_object()
     31 ip6_address_delete()
     32 nm_platform_ip6_address_delete()
     33 nm_platform_ip6_address_sync()
 >>> 34 nm_ip6_config_commit()
     35 ndisc_config_changed()
     36 ffi_call_unix64()
     37 ffi_call()
     38 g_cclosure_marshal_generic_va()
     39 _g_closure_invoke_va()
     40 g_signal_emit_valist()
     41 g_signal_emit()
     42 check_timestamps()
     43 receive_ra()
     44 ndp_call_eventfd_handler()
     45 ndp_callall_eventfd_handler()
     46 event_ready()
     47 g_main_context_dispatch()
     48 g_main_context_iterate.isra.22()
     49 g_main_loop_run()
 >>> 50 main()

NMPlatform already has a check to assert against recursive calls
in delayed_action_handle_all():

    g_return_val_if_fail (priv->delayed_action.is_handling == 0, FALSE);

    priv->delayed_action.is_handling++;
    ...
    priv->delayed_action.is_handling--;

Fixes: f85728ec

https://bugzilla.redhat.com/show_bug.cgi?id=1546656