fix rbenv() function in ksh and dash

ksh syntax becomes:

    function rbenv {
      typeset command

`typeset` only declares a local variable if there's an explicit
`function` declaration; otherwise the variable leaks.

Other shells use this syntax:

    rbenv() {
      local command

This is for dash compatibility, which supports neither `function` nor
`typeset`.

references #205, fixes #408

libexec/rbenv-init

@@ -122,13 +122,25 @@ function rbenv
     command rbenv "\$command" \$argv
   end
 end
+EOS
+  exit 0
+  ;;
+ksh )
+  cat <<EOS
+function rbenv {
+  typeset command
 EOS
   ;;
 * )
-  IFS="|"
   cat <<EOS
 rbenv() {
-  typeset command
+  local command
+EOS
+  ;;
+esac
+
+IFS="|"
+cat <<EOS
   command="\$1"
   if [ "\$#" -gt 0 ]; then
     shift
@@ -142,5 +154,3 @@ rbenv() {
   esac
 }
 EOS
-  ;;
-esac