openssl: Implement client-side NPN protocol selection

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.