readdir_r is deprecated

[mpartel]

Turns out readdir_r was deprecated around a year ago: https://lwn.net/Articles/696474/

We can probably safely replace it with readdir, but should check documentation on OS X and FreeBSD to be sure. Switching away from it would help avoid problems like #54.

[tyll]

readdir seems to exist both on OS X and FreeBSD:
https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man3/readdir.3.html
https://www.freebsd.org/cgi/man.cgi?query=readdir&sektion=3

[mpartel]

Sorry, I was vague. The question is whether we can count on it being thread-safe. Neither of those man-pages seems mention that.

I can easily imagine a thread-safe implementation and don't know why any non-ancient implementation wouldn't be thread-safe, but it'd be nice to know for sure.