[PATCH] Fix readdir_r with long file names

[Roland McGrath]

> There's precedent for my approach in realpath, where we don't use 
> pathconf(path, _PC_PATH_MAX), either, but cap the path length at PATH_MAX.

That's just following vanilla POSIX.1 rules: if PATH_MAX is defined at
compile time, it's a fixed system-wide limit.  If PATH_MAX is not defined
at compile time, there might or might not be a limit and if there is a
limit it might be filesystem-specific; pathconf reports that.