Restructure incomplete or broken "autoconf" implementations

Some plugins with non-zero autoconf exitcodes (it must be zero instead)
deserved a bit of code cleanup for improved readability.

plugins/arp/arp

@@ -15,10 +15,11 @@
 
 if [ "$1" = "autoconf" ]; then
     # Search for arp
-    which arp >/dev/null 2>/dev/null || (echo "no (can't find arp binary)" && exit 1)
-
-    # ...or success
-    echo yes
+    if which arp >/dev/null; then
+        echo yes
+    else
+        echo "no (missing 'arp' executable)"
+    fi
     exit 0
 fi
 

plugins/arp/arp_

@@ -19,13 +19,11 @@
 case "$1" in
 	autoconf)
 		# Search for ip
-		which ip >/dev/null 2>&1
-		if [ $? -ne 0 ]; then
-			echo "no (can't find ip binary)"
-			exit 1
+		if which ip >/dev/null; then
+			echo 'yes'
+		else
+			echo "no (missing 'ip' executable)"
 		fi
-		# ...or success
-		echo 'yes'
 		exit 0
 	;;
 	suggest)