Use ALPN for protocol negotiation over TLS

[Denis Smirnov]

As discussed on DCNF meeting, the ALPN proposal is at final stages:
https://github.com/direct-connect/protocol/pull/1
It may be time to add the prototype implementation to the DC++ client.

The example can be found in cURL source:
https://github.com/curl/curl/blob/master/lib/vtls/openssl.c
(see mentions of tls_enable_alpn flag).

The function to set supported protocols for a TLS context is SSL_CTX_set_alpn_protos.

[Denis Smirnov (dennwc)]

First part of the change for Flylink:
https://github.com/pavel-pimenov/flylinkdc-r5xx/pull/1720

[Denis Smirnov (dennwc)]

Proposed patch for advertising a list of supported protocols:
https://gist.github.com/dennwc/de336c848695232aab72c06b943349ab

[Denis Smirnov (dennwc)]

Same change for EiskaltDC++:
https://github.com/eiskaltdcpp/eiskaltdcpp/pull/396