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>

Several callers of EVP_EncodeBlock are doing ad-hoc versions of this
function without any overflow checks.

Change-Id: I4d0cad2347ea8c44b42465e8b14b2783db69ee8f
Reviewed-on: https://boringssl-review.googlesource.com/1511
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

When I switched the base64 code to use size_t, I missed that one of the
loops was counting down, not up, and depended on the loop variable going
negative.

Additionally this change fixes a bug in NETSCAPE_SPKI_b64_encode where
the size of the result buffer was incorrectly calculated and a possible
memory leak.

Change-Id: Ibdf644244291274f50b314f3bb13a61b46858ca1
Reviewed-on: https://boringssl-review.googlesource.com/1220
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)