At halt/killpower time, /usr may already be umounted, and wall
lives in /usr. Avoid failing by using cat if needed.

Index: apcupsd/platforms/apccontrol.in
===================================================================
--- apcupsd.orig/platforms/apccontrol.in
+++ apcupsd/platforms/apccontrol.in
@@ -62,10 +62,10 @@ fi
 
 case "$1" in
     killpower)
-	echo "Apccontrol doing: ${APCUPSD} --killpower on UPS ${2}" | ${WALL}
+	echo "Apccontrol doing: ${APCUPSD} --killpower on UPS ${2}" | (${WALL} 2>/dev/null || cat)
 	sleep 10
 	${APCUPSD} --killpower
-	echo "Apccontrol has done: ${APCUPSD} --killpower on UPS ${2}" | ${WALL}
+	echo "Apccontrol has done: ${APCUPSD} --killpower on UPS ${2}" | (${WALL} 2>/dev/null || cat)
     ;;
     commfailure)
 	echo "Warning communications lost with UPS ${2}" | ${WALL}