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>

Change-Id: Ie35b8d0e2da0f7d2588c4a436fc4b2b2596aaf18
Reviewed-on: https://boringssl-review.googlesource.com/7791
Reviewed-by: David Benjamin <davidben@google.com>

There was only one function that required BoringSSL to know how to read
directories. Unfortunately, it does have some callers and it's not immediately
obvious whether the code is unreachable. Rather than worry about that, just
toss it all into decrepit.

In doing so, do away with the Windows and PNaCl codepaths. Only implement
OPENSSL_DIR_CTX on Linux.

Change-Id: Ie64d20254f2f632fadc3f248bbf5a8293ab2b451
Reviewed-on: https://boringssl-review.googlesource.com/7661
Reviewed-by: Adam Langley <agl@google.com>

This reverts commit 112c4dd1ff. Accidentally used
the wrong push line.

There was only one function that required BoringSSL to know how to read
directories. Unfortunately, it does have some callers and it's not immediately
obvious whether the code is unreachable. Rather than worry about that, just
toss it all into decrepit.

In doing so, do away with the Windows and PNaCl codepaths. Only implement
OPENSSL_DIR_CTX on Linux.

Change-Id: I3eb55b098e3aa042b422bb7da115c0812685553e