Complete phase three of 3da1cf1e88f8.

In D39638, the values of SYSCTLs belong to VNETs will not reflect the change of kernel environment variable after the kernel modules been initialized. With this change new vnets will get latest values when corresponding kernel environment variables change.

MFC after: 3 weeks
Relnotes: yes

Test the following cases, on x86-64 and riscv hardwares:

set kernel environment variables, create new vnets, check sysctls
unset kernel environment variables, create new vnets, check sysctls

root@:~ # kldload -nq if_bridge
root@:~ # sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 0
root@:~ # kenv net.link.bridge.log_stp=1
net.link.bridge.log_stp="1"
root@:~ # jail -ic vnet persist
4
root@:~ # jexec 4 sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 1
root@:~ # sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 0
root@:~ # kenv -u net.link.bridge.log_stp
root@:~ # jexec 4 sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 1
root@:~ # jail -ic vnet persist
5
root@:~ # jexec 5 sysctl net.link.bridge.log_stp
net.link.bridge.log_stp: 0

The above sysctl net.link.bridge.log_stp is in kernel module if_bridge.

Repeat tests for sysctl net.inet.tcp.syncache.hashsize which is in kernel.