Add a simplified SSL BIO for curl.

A recent change to curl[1] added support for HTTPS proxies, which
involves running a TLS connection inside another TLS connection. This
was done by using SSL BIOs, which we removed from BoringSSL for being
crazy.

This change adds a stripped-down version of the SSL BIO to decrepit in
order to suport curl.

[1] https://github.com/curl/curl/commit/cb4e2be7c6d42ca0780f8e0a747cecf9ba45f151

Change-Id: I9cb8f2db5b28a5a70724f6f93544297c380ac124
Reviewed-on: https://boringssl-review.googlesource.com/12631
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index 58a4747..ef7e465 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -679,6 +679,8 @@
  * on one line. */
 OPENSSL_EXPORT const BIO_METHOD *BIO_f_base64(void);
 
+OPENSSL_EXPORT void BIO_set_retry_special(BIO *bio);
+
 
 /* Private functions */