Take advantage of normalized return values.

Now that much of EVP has been normalized to 0/1, a lot of code can just use
boolean operators. (As can some code which was already using them...)

Change-Id: I6bb17edfd6f67050bf1706d59d8f37df57535faa
Reviewed-on: https://boringssl-review.googlesource.com/3875
Reviewed-by: Adam Langley <agl@google.com>