Don't protect savecore(8) from being run with a bad dumpdir

argument. Don't fail silently, but let savecore(8) make noise. It
won't behave badly, it doesn't need protection.

At the same time, allow the administrator to have dumpdev enabled
while dumpdir (savecore(8)) is disabled and document how to do it.

PR:		conf/35725

etc/rc

@@ -552,12 +552,17 @@ case ${dumpdev} in
 	'')
 		dumpdir='/var/crash'
 		;;
+	[Nn][Oo])
+		dumpdir='NO'
+		;;
 	esac
 
-	if [ -e "${dumpdev}" -a -d "${dumpdir}" ]; then
+	if [ -e "${dumpdev}" ]; then
 		/sbin/dumpon -v ${dumpdev}
-		echo -n 'Checking for core dump: '
-		/sbin/savecore ${savecore_flags} "${dumpdir}"
+		if [ "$dumpdir" != 'NO' ]; then
+			echo -n 'Checking for core dump: '
+			/sbin/savecore ${savecore_flags} "${dumpdir}"
+		fi
 	fi
 	;;
 esac

share/man/man5/rc.conf.5

@@ -1727,6 +1727,13 @@ specified by the
 variable.
 The default value is
 .Dq /var/crash .
+Set to
+.Dq NO
+to not run
+.Xr savecore 8
+at boot time when
+.Va dumpdir
+is set.
 .It Va enable_quotas
 .Pq Vt bool
 Set to