Re-work the OpenSSL portability header to be a portability header

Re-work the OpenSSL portability header to be a portability header

Summary:
It was defining the same functions as OpenSSL, but it was putting them in the `folly::ssl` namespace and requiring a manual `use namespace` everywhere to actually be able to use it. Proper portability headers require no interaction to function correctly other than including them in the build.
This adjusts it so including it is all that is required to get an OpenSSL 1.1.0 compatible API.
This also re-organizes the code to significantly lower the amount of nesting in the `#if`'s, and puts it in order from oldest APIs to newest, rather than haphazard.

Reviewed By: yfeldblum

Differential Revision: D4856784

fbshipit-source-id: 7c00fdf4a3e51e37287d2b1ef294046f38a96a43