network: make resolving NetDev names delayed and moved to network_verify()

And before resolving NetDev names, check conditions in .network,
and if they do not match the system environment, drop the network
unit earlier.

Fixes #4211.

src/network/netdev/netdev.h

@@ -47,6 +47,7 @@ typedef enum NetDevKind {
         NETDEV_KIND_FOU,
         NETDEV_KIND_ERSPAN,
         _NETDEV_KIND_MAX,
+        _NETDEV_KIND_TUNNEL, /* Used by config_parse_stacked_netdev() */
         _NETDEV_KIND_INVALID = -1
 } NetDevKind;