Make warning configurable for obsolete per-jail configuration

[dteske]

Summary

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).

Test Plan

Setup per-jail configurations in an rc.conf(5) style.
See https://svnweb.freebsd.org/base/head/share/examples/jails/README

% service jail restart

You'll get the following warning at the end...

/etc/rc.d/jail: WARNING: Per-jail configuration via jail_* variables \
is obsolete. Please consider migrating to /etc/jail.conf.

% sysrc jail_confwarn=NO
% service jail restart

The warning should no longer appear.

[julian]