openssl: Add method for setting protocols to be used in NPN

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.