Turns out the EVP_CIPH_FLAG_CUSTOM_CIPHER ciphers (i.e. legacy EVP_CIPHER
AES-GCM) have a completely different return value setup than the normal ones
which are the standard one/zero. (Except that they never return zero; see
TODO.)

Fix checks in ssl/ and remove remnants of EVP_CIPH_FLAG_CUSTOM_CIPHER in ssl/
as we're using EVP_AEAD now.

See CHANGES entry added in upstream's 3da0ca796c.

Change-Id: Ia4d0ff59b03c35fab3a08141c60b9534cb7172e2
Reviewed-on: https://boringssl-review.googlesource.com/2606
Reviewed-by: Adam Langley <agl@google.com>