Start providing parts of the OpenSSL 1.1 API.

Start providing parts of the OpenSSL 1.1 API.

This will ease the burden on ports and others trying to make software
work with LibreSSL, while avoiding #ifdef mazes. Note that we are not
removing 1.0.1 API or making things opaque, hence software written to
use the older APIs will continue to work, as will software written to
use the 1.1 API (as more functionality become available).

Discussed at length with deraadt@ and others.