Fortify against missing parameters

If user does not provide mandatory parameter, `shift 2` will fail but
will not shift anything, resulting in infinite loop.

src/libexec/saturnin-dmenu

 field=2-
 
 while true; do case $1 in
-    -l|--lines)     lines=$2; shift 2 ;;
-    --split)        delim=$2; maybe_split=do_split; shift 2 ;;
-    --split-field)  field=$2; maybe_split=do_split; shift 2 ;;
+    -l|--lines)     lines=$2; shift 2 || usage ;;
+    --split)        delim=$2; maybe_split=do_split; shift 2 || usage ;;
+    --split-field)  field=$2; maybe_split=do_split; shift 2 || usage ;;
     --help|--usage) usage             ;;
     *)              break             ;;
 esac done

src/libexec/saturnin-ln

 }
 
 while true; do case $1 in
-    -t|--target)        target=$2           shift 2 ;;
+    -t|--target)        target=$2           shift 2 || usage ;;
     --help)             usage                       ;;
     -*)                 usage                       ;;
     *)                  src=$1; name=$2;    break   ;;

src/libexec/saturnin-uris

 
 src=args
 while true; do case "$1" in
-    -f) src=file; file=$2; shift 2 ;;
+    -f) src=file; file=$2; shift 2 || usage ;;
     -1) src=xpri;          shift   ;;
     -2) src=xsec;          shift   ;;
     -c) src=xclp;          shift   ;;