Add support for SNI

[mweinelt]

There are use cases where multiple certificates are required to serve matching hostnames.

It will probably be part of ircv3.3 if that is ever completed.
https://ircv3.net/specs/core/sni-3.3.html

There is already widespread client support:

• Weechat: weechat/weechat@136da36
• HexChat: hexchat/hexchat@dbad928
• irssi: irssi/irssi@28aaa65
• znc: znc/znc@47f887c

[Mikaela]

Could eSNI be supported too?

[mweinelt]

Eagerly waiting for @ManiacTwister to submit hackint's implementation for SNI support.