Many shops still prefer rc.conf(5) based jail configuration(s). In-part
because they can use sysrc in conjunction with ssh and xargs to perform
en-masse changes in a large distribution with lots of jails spread over
many hosts on a LAN/WAN.
Provide a mechanism for disabling the warning eschewed by /etc/rc.d/jail
in said situation. If jail_confwarn="NO" is in rc.conf(5) (default "YES")
skip the warning that per-jail configurations are obsolete and that the
user should migrate to jail.conf(5).