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

[Paul Eggert]

Attached is a patch that can greatly improve the performance of
directory-files-and-attributes, which I'd like to install
into the trunk.  The key idea is to use fstatat
instead of lstat; this reduces directory-reading
from an O(ND) to an O(N) operation, where N is the number
of directory entries and D is the length of
the directory's name.

This patch works on POSIXish hosts, but it'll no doubt
need a few changes to the MS-Windows makefiles to compile
fdopendir.c, fstatat.c, openat-die.c, save-cwd.c, openat-proc.c,
or whatever subset of these is needed on MS-Windows.
I'll CC: this to Eli to give him a heads-up.

This patch is relative to trunk bzr 111595.

fdopendir.txt
Description: Text document