ssl: Rework our custom BIO type
ssl: Rework our custom BIO type

This commit changes to an actual new BIO method rather than reusing an
existing BIO method, and overriding only the fields that we need.
The approach before this commit would be causing issues with OpenSSL
1.1.0 as some of the fields we access have become opaque.

flexVDI/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk