dumpdev: respect kenv for stable branches

We have somewhat twisted logic to determine actions for dumpdev
considering three sources of information:

* kenv "dumpdev" tunnable supposed to point to specific device;
* /etc/defaults/rc.conf "dumpdev" variable;
* /etc/rc.conf that may be unset or set to "NO", "AUTO" or device name.

For CURRENT without any setting in kenv or /etc/rc.conf
the default is "AUTO". For STABLE branches the default is "NO".

Current implementation breaks for STABLE branches if kenv points
to specific device but /etc/rc.conf does not set "dumpdev" at all.

Let us fix it commenting out "dumpdev" in /etc/defaults/rc.conf
for STABLE branches and making the code to consult kenv
if "dumpdev" is not set elsewhere.

MFC-after:	1 month

libexec/rc/rc.conf

@@ -643,7 +643,9 @@ lpd_flags=""		# Flags to lpd (if enabled).
 nscd_enable="NO"	# Run the nsswitch caching daemon.
 chkprintcap_enable="NO"	# Run chkprintcap(8) before running lpd.
 chkprintcap_flags="-d"	# Create missing directories by default.
-dumpdev="AUTO"		# Device to crashdump to (device name, AUTO, or NO).
+dumpdev="AUTO"		# Device to crashdump to (device name, AUTO, or NO);
+			# this should be commented out here
+			# for stable branches to respect kenv.
 dumpon_flags=""		# Options to pass to dumpon(8), followed by dumpdev.
 dumpdir="/var/crash"	# Directory where crash dumps are to be stored
 savecore_enable="YES"	# Extract core from dump devices if any

libexec/rc/rc.d/dumpon

@@ -52,15 +52,18 @@ dumpon_start()
 	# early so a crash early in the boot process can be caught.
 	#
 	case ${dumpdev} in
-	[Nn][Oo] | '')
+	[Nn][Oo])
 		;;
-	[Aa][Uu][Tt][Oo])
+	[Aa][Uu][Tt][Oo] | '')
 		root_hold_wait
 		dev=$(/bin/kenv -q dumpdev)
 		if [ -n "${dev}" ] ; then
 			dumpon_try "${dev}"
 			return $?
 		fi
+		if [ -z ${dumpdev} ] ; then
+			return
+		fi
 		while read dev mp type more ; do
 			[ "${type}" = "swap" ] || continue
 			case ${dev} in
@@ -85,7 +88,7 @@ dumpon_start()
 dumpon_stop()
 {
 	case ${dumpdev} in
-	[Nn][Oo] | '')
+	[Nn][Oo])
 		;;
 	*)
 		rm -f /dev/dumpdev

libexec/rc/rc.d/savecore

@@ -20,11 +20,11 @@ savecore_prestart()
 {
 	# Quit if we have no dump device
 	case ${dumpdev} in
-	[Nn][Oo] | '')
+	[Nn][Oo])
 		debug 'No dump device. Quitting.'
 		return 1
 		;;
-	[Aa][Uu][Tt][Oo])
+	[Aa][Uu][Tt][Oo] | '')
 		if [ ! -L /dev/dumpdev ]; then
 			return 1
 		fi