readdir and readdir_r have a sad history:
https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html
https://womble.decadent.org.uk/readdir_r-advisory.html
http://austingroupbugs.net/view.php?id=696

Martin Thomson reports that newer glibcs warn that readdir_r is
deprecated. Especially since this has been banished to libdecrepit
anyway, go ahead and honor that warning. OpenSSL also uses readdir, so
we're no worse than they are.

While I'm here, rewrite this to remove a useless layer of abstraction,
now that we've punted on supporting most platforms here. Also remove the
redundant documentation comment (there's one in the header already).

Change-Id: I5350c55417a7f5c4c4725f97dd63f960aeb96801
Reviewed-on: https://boringssl-review.googlesource.com/11220
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>