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

etc/mtree/BSD.root.dist

@@ -56,6 +56,8 @@
         ..
         gss
         ..
+        jail.conf.d
+        ..
         kyua             tags=package=tests
         ..
         mail

libexec/rc/rc.d/jail

@@ -120,12 +120,16 @@ parse_options()
 	fi
 	eval _jconf=\"\${jail_${_jv}_conf:-/etc/jail.${_j}.conf}\"
 	eval _rootdir=\"\$jail_${_jv}_rootdir\"
+	eval _jconfdir=\"/etc/jail.conf.d/${_j}.conf\"
 	eval _hostname=\"\$jail_${_jv}_hostname\"
 	if [ -z "$_rootdir" -o \
 	     -z "$_hostname" ]; then
 		if [ -r "$_jconf" ]; then
 			_conf="$_jconf"
 			return 0
+		elif [ -r "$_jconfdir" ]; then
+			_conf="$_jconfdir"
+			return 0
 		elif [ -r "$jail_conf" ]; then
 			_conf="$jail_conf"
 			return 0

share/man/man5/rc.conf.5

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 8, 2021
+.Dd September 10, 2021
 .Dt RC.CONF 5
 .Os
 .Sh NAME
@@ -3883,6 +3883,13 @@ The configuration filename used by
 utility.
 The default value is
 .Pa /etc/jail.conf .
+.Pa /etc/jail.  Ns Ao Ar jname Ac Ns Va .conf
+and
+.Pa /etc/jail.conf.d/ Ns Ao Ar jname Ac Ns Va .conf
+will also be used if
+.Va Ao Ar jname Ac Va
+is set in
+.Va jail_list .
 .It Va jail_parallel_start
 .Pq Vt bool
 If set to