Fix arbitrary code execution in printf invocation

Without `--`, printf might continue searching for options.

A known exploit is shown in http://wiki.bash-hackers.org/commands/builtin/printf#options.

quinedb

@@ -14,18 +14,18 @@ print_db(){
     echo "db=("
     i=0
     for k in "${!db[@]}"; do
-        escaped_keys[$i]=$(printf %q "$k")
+        escaped_keys[$i]=$(printf %q -- "$k")
         i=$((i+1))
     done
 
     # sort the keys for deterministic printing
-    IFS=$'\n' sorted=($(printf '%s' "${escaped_keys[*]}" | sort))
+    IFS=$'\n' sorted=($(printf '%s' -- "${escaped_keys[*]}" | sort))
     unset IFS
 
     for k in "${sorted[@]}"; do
         unescaped=$(eval "echo $k")
         v=${db["$unescaped"]}
-        echo "    [$k]=$(printf %q "$v")"
+        echo "    [$k]=$(printf %q -- "$v")"
     done
     echo ")"
 }
@@ -47,18 +47,18 @@ print_db(){
     echo "db=("
     i=0
     for k in "${!db[@]}"; do
-        escaped_keys[$i]=$(printf %q "$k")
+        escaped_keys[$i]=$(printf %q -- "$k")
         i=$((i+1))
     done
 
     # sort the keys for deterministic printing
-    IFS=$'\n' sorted=($(printf '%s' "${escaped_keys[*]}" | sort))
+    IFS=$'\n' sorted=($(printf '%s' -- "${escaped_keys[*]}" | sort))
     unset IFS
 
     for k in "${sorted[@]}"; do
         unescaped=$(eval "echo $k")
         v=${db["$unescaped"]}
-        echo "    [$k]=$(printf %q "$v")"
+        echo "    [$k]=$(printf %q -- "$v")"
     done
     echo ")"
 }
@@ -89,7 +89,7 @@ case "$1" in
     "get")
         if [ "${db["$2"]+_}" ]; then
             v=${db["$2"]}
-            echo "$(printf %q "$v")" >&2
+            echo "$(printf %q -- "$v")" >&2
         fi
     ;;
     "set")
@@ -101,7 +101,7 @@ case "$1" in
         echo 'OK' >&2
     ;;
     "keys")
-        for k in "${!db[@]}"; do echo "$(printf %q "$k")"; done >&2
+        for k in "${!db[@]}"; do echo "$(printf %q -- "$k")"; done >&2
     ;;
     *)
         echo "USAGE: quinedb [get k | set k v | delete k | keys]" >&2
@@ -135,7 +135,7 @@ case "$1" in
     "get")
         if [ "${db["$2"]+_}" ]; then
             v=${db["$2"]}
-            echo "$(printf %q "$v")" >&2
+            echo "$(printf %q -- "$v")" >&2
         fi
     ;;
     "set")
@@ -147,7 +147,7 @@ case "$1" in
         echo 'OK' >&2
     ;;
     "keys")
-        for k in "${!db[@]}"; do echo "$(printf %q "$k")"; done >&2
+        for k in "${!db[@]}"; do echo "$(printf %q -- "$k")"; done >&2
     ;;
     *)
         echo "USAGE: quinedb [get k | set k v | delete k | keys]" >&2