Add support for jail.conf.d

Using /etc/jail.{jailname}.conf is nice, however it makes /etc/ very
messy if you have many jails. This patch allows one to move these
config files out of the way into /etc/jail.conf.d/{jailname}.conf.

Note that the same caveat as /etc/jail.*.conf applies: the jail service
will not autodiscover all of these for starting 'all' jails. This is
considered future work, since the behavior matches.

Reviewed by: kevans
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D24570