MFC r344794:

tun: VIMAGE fix for if_tun cloner

The if_tun cloner is not virtualised, but if_clone_attach() does use a
virtualised list of cloners.
The result is that we can't find the if_tun cloner when we try to remove
a renamed tun interface. Virtualise the cloner, and move the final
cleanup into a sysuninit so that we're sure this happens after all of
the vnet_sysuninits

Note that we need unit numbers to be system-unique (rather than unique
per vnet, as is done by if_clone_simple()). The unit number is used to
create the corresponding /dev/tunX device node, and this node must match
with the interface.
Switch to if_clone_advanced() so that we have control over the unit
numbers.

Reproduction scenario:
jail -c -n foo persist vnet
jexec test ifconfig tun create
jexec test ifconfig tun0 name wg0
jexec test ifconfig wg0 destroy

PR: 235704
Reviewed by: bz, hrs, hselasky
Differential Revision: https://reviews.freebsd.org/D19248