After the `set_npn_protocols` method of the `SslContext` struct is
called, any future TLS connections established with this context will
perform NPN negotiation.
The chosen protocol is the one with the highest priority in the
server's protocol list that is also in the client's protocol list.
(This is the default behavior provided by OpenSSL's
`SSL_select_next_proto` function.)
If there is no overlap between the two lists, no error is raised.
A new method `set_npn_protocols` is added to the `SslContext` struct,
when the `npn` feature is enabled.
The method takes a list of protocols that are supported by the peer.
These protocols will be used during Next Protocol Negotiation.
The method saves the given list within the extra data of the OpenSSL
Context structure, so that the list can be referred to later on by the
callbacks invoked during TLS connection establishment.
Marko Lalic