Define ALPN

[pthatcherg]

QUIC requires an ALPN value, so we should pick one and register it with IANA.

How about "osp"?

[markafoltz]

We will need to register it with IANA:

https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids

[markafoltz]

PR #229 adds "osp" to the ALPN to the spec. I'll close this one and open a new issue for IANA registration since that's not related to the spec itself.