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).

Reviewed by: jelischer
MFC after: 3 days
Sponsored by: FIS Global, Inc.
Differential Revision: https://reviews.freebsd.org/D7465