Implementations of ENGINEs often don't want to implement every function.
This change adds an error code for those situations.

Change-Id: Id6b7eace36d06ffad7f347f556d942d447d8a2fd
Reviewed-on: https://boringssl-review.googlesource.com/1920
Reviewed-by: Wan-Teh Chang <wtc@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Previously, public headers lived next to the respective code and there
were symlinks from include/openssl to them.

This doesn't work on Windows.

This change moves the headers to live in include/openssl. In cases where
some symlinks pointed to the same header, I've added a file that just
includes the intended target. These cases are all for backwards-compat.

Change-Id: I6e285b74caf621c644b5168a4877db226b07fd92
Reviewed-on: https://boringssl-review.googlesource.com/1180
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.)