Move executable-check to C++

This was already apparently supposed to work, but didn't because we
just overrode errno again.

This now means that, if a correctly named candidate exists, we don't
start the command-not-found handler.

See #8804

share/config.fish

@@ -13,12 +13,6 @@ or set -g __fish_added_user_paths
 #
 function __fish_default_command_not_found_handler
     printf (_ "fish: Unknown command: %s\n") (string escape -- $argv[1]) >&2
-    for file in $PATH/$argv[1]
-        if test -e $file -a ! -x $file
-            printf (_ "fish: %s exists but isn't executable\n") (string escape -- $file) >&2
-            break
-        end
-    end
 end