cleanup: fix readdir_r() usage

[garlick]

readdir_r() is used in various places throughout flux-core, but may be unnecessary and is likely to be deprecated in a future POSIX version.

The allocation of the (buffer) dirent is done on the stack in Flux, but this is not the recommended method according to readdir_r(3) (see NOTES section).

Further, the use of readdir_r() may not be necessary, since readdir() is thread safe across directory streams. Within the stream, a recursive walk can be accomplished with readdir() as long as the returned entry is not used after recursion (or is copied beforehand).

See also: https://lwn.net/Articles/696474/

[grondo]

Hm, had this cleanup done on the extensor branch I think. I could probably pull those out into a separate PR.

[garlick]

Not urgent, but welcome.