Eliminate unnecessary includes from low-level crypto modules.

Beyond generally eliminating unnecessary includes, eliminate as many
includes of headers that declare/define particularly error-prone
functionality like strlen, malloc, and free. crypto/err/internal.h was
added to remove the dependency on openssl/thread.h from the public
openssl/err.h header. The include of <stdlib.h> in openssl/mem.h was
retained since it defines OPENSSL_malloc and friends as macros around
the stdlib.h functions. The public x509.h, x509v3.h, and ssl.h headers
were not changed in order to minimize breakage of source compatibility
with external code.

Change-Id: I0d264b73ad0a720587774430b2ab8f8275960329
Reviewed-on: https://boringssl-review.googlesource.com/4220
Reviewed-by: Adam Langley <agl@google.com>

crypto/directory_win.c

diff --git a/crypto/directory_win.c b/crypto/directory_win.c
index 2e4600f..4ebacf2 100644
--- a/crypto/directory_win.c
+++ b/crypto/directory_win.c
@@ -41,6 +41,9 @@
 #define NAME_MAX 255
 #endif
 
+#include <openssl/mem.h>
+
+
 struct OPENSSL_dir_context_st {
   WIN32_FIND_DATA ctx;
   HANDLE handle;