Partially revert VNET change and expand VNET structure.

Revert parts of r353274 replacing vnet_state with a shutdown flag.

Not having the state flag for the current SI_SUB_* makes it harder to debug
kernel or module panics related to VNET bringup or teardown.
Not having the state also does not allow us to check for other dependency
levels between components, e.g. for moving interfaces.

Expand the VNET structure with the new boolean flag indicating that we are
doing a shutdown of a given vnet and update the vnet magic cookie for the
change.

Update libkvm to compile with a bool in the kernel struct.

Bump __FreeBSD_version for (external) module builds to more easily detect
the change.

Reviewed by: hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D23097