Make saturnin-ln more "helpful" on bad syntax

src/libexec/saturnin-ln

 while true; do case $1 in
     -f|--force)         FORCE=true;         shift   ;;
     -t|--target)        target=$2           shift 2 ;;
+    --help)             usage                       ;;
+    -*)                 usage                       ;;
     "")                 usage                       ;;
     *)                  src=$1; name=$2;    break   ;;
 esac done