This ensures that all the Rust functions, types and constants
always match the actual BoringSSL definitions.
It also removes quite a lot of manually maintained code, as well
as the need for systest.
The value for `SslOptions::ALL`, for example, was wrong. On current
BoringSSL versions, this is a no-op, and is set to `0`.
Clearing it does nothing. So, the `clear_ctx_options` test, that
passed by accident, was adjusted to use a different option.
The `libc` crate is not required, as we only use it for types that
are already defined in the standard library. It was removed from
`boring-sys`. The same can be done to other crates later.
Frank Denis