If commit_mtu() is called multiple times and dev->get_configured_mtu()
returns @is_user_config=FALSE, only the first call changes the
MTU. So, for example, when the parent MTU of a VLAN changes, we apply
the new MTU only the first time.

Rework the handling of MTU in NMDevice, and store the source of the
configured MTU. When commit_mtu() is called again, we ask the subclass
a MTU to configure and apply it only if the source has higher
priority, or when the parent MTU changed.