service: Always include start and stop in service command completions.

Not all services are in /etc/init.d files (such as systemd ones) from
which command completions can be parsed from.

completions/service

@@ -22,7 +22,7 @@ _service()
             sysvdir=/etc/rc.d/init.d || sysvdir=/etc/init.d
         COMPREPLY=( $( compgen -W '`sed -e "y/|/ /" \
             -ne "s/^.*\(U\|msg_u\)sage.*{\(.*\)}.*$/\2/p" \
-            $sysvdir/${prev##*/} 2>/dev/null`' -- "$cur" ) )
+            $sysvdir/${prev##*/} 2>/dev/null` start stop' -- "$cur" ) )
     fi
 
     return 0