Add a "vnet_state" field to struct vnet.
This is set to the SI_SUB_* value before executing any VNET_SYSINIT
or VNET_SYSUNINT. While good for debugging especially VNET teardown
problems having a chance to know at which level during teardown we are,
it will also be used to identify to detcted a "stable state"
(as in fully up and running) later on.

Obtained from: projects/vnet
Sponsored by: The FreeBSD Foundation