bug#13539: Use fdopendir, fstatat and readlinkat, for efficiency.

[Stefan Monnier]

>> perhaps a further optimization would be to avoid the call to
>> file_name_completion_stat if completion-ignored-extensions is devoid
>> of elements that end in a slash.
> Yes, something like that might make sense.

Maybe.  But maybe not.  The default value of
completion-ignored-extensions includes elements that end in a slash, and
I think fairly few uses of try-completion rebind it to something else.


        Stefan