Fix the return values for most of SRTP.

Switch all of SRTP code to the standard return value convention with two
exceptions. Unfortunately, OpenSSL exposed API with the wrong error code. Keep
the public API flipped and document.

Change-Id: I43ac82513f4f52bb36a0b54aba9b9e0fa285730e
Reviewed-on: https://boringssl-review.googlesource.com/1691
Reviewed-by: Adam Langley <agl@google.com>
3 files changed