Add preliminary ALPN support for OpenSSL backend

[seanmonstar]

No description provided.

[seanmonstar]

Seems CI failures are unhappy about lazy_static 0.2.

[sfackler]

I don't really want to support a feature that only works on one backend - the whole point of this crate is that it'll work anywhere!

[sfackler]

The sequence that openssl wants is literally the byte length of the protocol followed by it, not the string \xN.

[seanmonstar]

Ah, my eyes tricked me when reading https://github.com/sfackler/hyper-openssl/blob/master/src/lib.rs#L107.

[seanmonstar]

I assumed as mentioned in #49, support for backends could be added in as they exist.

[sfackler]

All of the underlying system APIs support ALPN.

[seanmonstar]

I don't see anything about ALPN in the schannel docs...

[sfackler]

steffengy/schannel-rs#49

[seanmonstar]

So to clarify, you'd rather not provide the API in native-tls until all 3 backends have the features available? I figured we could add support individually, since it's mostly an optimization, and things should still work if not available.

[sfackler]

So to clarify, you'd rather not provide the API in native-tls until all 3 backends have the features available?

Yep.