It's not properly implemented, to change the route-metric while
being active. That would require pecular re-initializations.

Avoid that a configuration reload, can change the used route-metric
unexpectedly.

It doesn't cover all potential changes, because
nm_manager_device_route_metric_reserve() might still return a different
value. That is however rather unlikely, because neither the ifindex nor
the device-type is expected to change.

NMConfig has pointers to current NMConfigData. The NMConfigData itself
is immutable, during config-reload, NMConfig will switch the instance
to a new config-data. This conveniently allows to take a snapshot
of a particular configuration.

Cache such an instance in the NMDevice, and initialize it, when
activation begins.

It's complicated to handle, and currently not handled properly.
Don't allow it, and fail.

Rather trivial change. Return-early, to completely handle the simpler
case (the success case) first. In the failure case, we only need
extra effort to generate a nice error message.

Proxy setting has not property NM_SETTING_IP_CONFIG_ROUTE_TABLE.
It's odd to handle it in the same if-block as IP configs.

Also, assign *_pp before unref-ing the old value. Calling
g_object_unref() on the old value, might invoke some callbacks
which are out of the contorl of nm_g_object_ref_set(). During
that time, the pointer should already be assigned the new value,
instead of having an intermediate %NULL value.

Also, don't use typeof(**_pp), which would not work with opaque
types (that we commonly have).

Fixes: bf3b3d444c7750a68a0771d99f48e663815e258e

https://github.com/NetworkManager/NetworkManager/pull/132