ALPN support for GnuTLS connections

[Eric Marsden <eric.marsden@risk-engineering.org>]

Hello,

The GnuTLS support in Emacs does not seem to support the TLS extension
Application Layer Protocol Negotiation (ALPN). ALPN is no longer just useful for
faster TLS handshakes (in HTTP/2, for example); it is mandatory in certain uses
of TLS.

The GnuTLS library does support ALPN (since 2013, it seems). My understanding is
that definitions for the two functions described here would need to be added to
gnutls.c:

   https://www.gnutls.org/manual/html_node/Application-Layer-Protocol-Negotiation-_0028ALPN_0029.html


Use case: the recent 17.0 release of PostgreSQL has added a "direct TLS"
connection mode which requires ALPN. Some hosted PostgreSQL providers only offer
direct TLS connections (I presume this allows them to use standard TLS
gateways). I would like to allow connections to these services using the pg-el
library (https://github.com/emarsden/pg-el), which implements the PostgreSQL
wire protocol.