Conscrypt will need these functions. Also fix a bug in
X509_get_extension_flags's error-handling. While I'm here, add
X509_CRL_get0_extensions for completeness. Nothing uses this yet, but
this could later be an alternative to avoid Conscrypt's mess with
templates.
Change-Id: I9393b75fcf53346535e6a4712355be081baa630d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/42744
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>