device: schedule an idle commit when setting device's sys-iface-state

When assuming a device, the NMActiveConnection switches the
sys-iface-state from "assume" to "managed" when the device reaches the
activated state.

  <debug> [1679353062.8884] active-connection[000055bd310b92e0]: set state activated (was activating)
  <debug> [1679353062.8885] active-connection[000055bd310b92e0]: update activation type from assume to managed

Note that the "assume" state is probably a misfeature, and should be
dropped in favor of more appropriate flags. Meaning, "assume" state for
the most part is very similar to sys-iface-state "managed", and the
cases where (during activation) we need to be graceful, may be better
covered with other (more specialized) state flags. Regardless, for most
practical purposes, sys-iface-state "assume" should be treated similar
to "managed" state.

When we fully activated, we should be sure to do yet another idle
commit. Note that scheduling an idle-commit is something that must
always be allowed to any users of NML3Cfg. The users have no knowledge
about each other and coordinate by registering their commit type
handles.  Issuing an idle "auto" commit must be therefore allowed to
them at any time.  If that were not the case, then there would be a bug
to fix. The only reason to maybe not do it, is when we are sure there is
nothing to commit and we would want to avoid unnecessary work.

You can easily reproduce this and see that we don't in fact schedule a
commit after becoming managed. A commit usually only happens later, for
example when we receive an autoconf6 update.

This affects for example setting the MTU. Currently, _commit_mtu() bails
out for nm_device_sys_iface_state_is_external_or_assume() and thus
during activation the MTU will not be set. Later, once we reach
activated state, due to this it still is not set right away. This patch
fixes that, although we should also change _commit_mtu() to not bail out
for sys-iface-state "assume".

src/core/devices/nm-device.c

@@ -3043,6 +3043,7 @@ nm_device_sys_iface_state_set(NMDevice *self, NMDeviceSysIfaceState sys_iface_st
               nm_device_sys_iface_state_to_string(sys_iface_state));
         priv->sys_iface_state_ = sys_iface_state;
         _dev_l3_cfg_commit_type_reset(self);
+        nm_device_l3cfg_commit(self, NM_L3_CFG_COMMIT_TYPE_AUTO, FALSE);
     }
 
     /* this function only sets a flag, no immediate actions are initiated.