I have wanted a utility that does exactly this. install -C does something similar, but has other stuff I don’t want. So far I just end up scripting cmp(1) to do the check.

I suggest we start with a switch which enables the new solution. That means it does not break stuff for people who upgrade.

In D38826#887279, @dvl wrote:

All jails.conf jail (in /etc/jail.conf, /etc/jail.*.conf and /etc/jail.conf.d/*.conf) start automatically, without the need to define them in jail_list in rc.conf

That will require an UPDATING notice.

AND: perhap a switch[es] to preserve the old behavior and if set, enable the new behavior.

This is my first attempt at a code contribution to FreeBSD... hopefully I'm going about it the right way.