paylad extracts in dedicated directory

the payload now extracts into either /tmp/tarinstall or
$XDG_CACHE_HOME/tarinstall in order to avoid conflicts

posix/payload

@@ -4,9 +4,9 @@
 # TODO: extract self without resolving to $0 because it doesn't resolve links
 
 if [ -z "$XDG_CACHE_HOME" ]; then
-	destdir=/tmp
+	destdir=/tmp/tarinstall
 else
-	destdir="$XDG_CACHE_HOME"
+	destdir="$XDG_CACHE_HOME"/tarinstall
 fi
 
 if ! [ "$(head -n 1 "$destdir"/ID/checksum 2>/dev/null)" = 'SUM' ]; then