Allow PKCS#8 in d2i_AutoPrivateKey.

This got lost in the initial commit. Add a test for d2i_AutoPrivateKey.

BUG=crbug.com/428671

Change-Id: Ib4f6114b03536edcfe3b1720a513f57f748e81d2
Reviewed-on: https://boringssl-review.googlesource.com/2130
Reviewed-by: Adam Langley <agl@google.com>