Lowercase dirnames in pd to avoid env variables

This is not foolproof, but most critical environment variables are all
uppercase, so lowercase variables will be mostly safe.

.zshrc

@@ -44,7 +44,7 @@ pd(){
 	else
 		pushd "$@"
 	fi
-	namedir $(echo $(basename "$PWD" | tr -cd 'A-Za-z0-9_') | sed -e 's/^[0-9]/_&/')
+	namedir $(echo $(basename "$PWD" | tr -cd 'A-Za-z0-9_') | tr 'A-Z' 'a-z' | sed -e 's/^[0-9]/_&/')
         echo "$PWD"
 }