jail: avoid a clash with /etc/jail.conf.d between rc and jail(8)

Since 13.1, /etc/rc.d/jail has looked for a per-jail config file in
/etc/jail.conf.d. For RELENG 14, the ".include" directive was added to
jail(8), with a sample line in the jail.conf(5) man page that includes
"/etc/jail.conf.d/*.conf".

These two use cases don't work together. When the jail.conf.d files
are included from a master jail.conf, the files in jail.conf.d are
likely to hold only partial configurations, and shouldn't be directly
loaded by rc.d/jail. But there are existing configurations that depend
on the current rc.d behavior. While users could be advised not to
include from /etc/jail.conf.d, it's the natural choice even if not
mentioned in jail.conf.5.

The workaround is for rc.d/jail to continue to load the individual
files, but only when /etc/jail.conf doesn't include from that
directory (via a simple grep test), This allows the current use
while not breaking the previous use.
Reported by:	antranigv at freebsd.am
Differential Revision:	https://reviews.freebsd.org/D41962

libexec/rc/rc.d/jail

@@ -126,7 +126,9 @@ parse_options()
 		if [ -r "$_jconf" ]; then
 			_conf="$_jconf"
 			return 0
-		elif [ -r "$_jconfdir" ]; then
+		elif [ -r "$_jconfdir" ] && ! egrep -q \
+		    '^\s*\.include\s*["'\'']?/etc/jail.conf.d/' "$jail_conf" \
+		    2>/dev/null; then
 			_conf="$_jconfdir"
 			return 0
 		elif [ -r "$jail_conf" ]; then