Introduce EVP_DecodeBase64.

This fixes several of the problems with the old API.
- Padding was completely ignored.
- ='s in the middle of the input were accepted.
- It tries to be helpful and strips leading/trailing whitespace.

Change-Id: I99b9d5e6583f7eaf9bf0b6ee9ca39799811b58dc
Reviewed-on: https://boringssl-review.googlesource.com/1602
Reviewed-by: Adam Langley <agl@google.com>
4 files changed