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] cb4e2be7c6
Change-Id: I9cb8f2db5b28a5a70724f6f93544297c380ac124
Reviewed-on: https://boringssl-review.googlesource.com/12631
Reviewed-by: Adam Langley <agl@google.com>
