HTTP/2: "http2" directive.

HTTP/2: "http2" directive.

The directive enables HTTP/2 in the current server.  The previous way to
enable HTTP/2 via "listen ... http2" is now deprecated.  The new approach
allows to share HTTP/2 and HTTP/0.9-1.1 on the same port.

For SSL connections, HTTP/2 is now selected by ALPN callback based on whether
the protocol is enabled in the virtual server chosen by SNI.  This however only
works since OpenSSL 1.0.2h, where ALPN callback is invoked after SNI callback.
For older versions of OpenSSL, HTTP/2 is enabled based on the default virtual
server configuration.

For plain TCP connections, HTTP/2 is now auto-detected by HTTP/2 preface, if
HTTP/2 is enabled in the default virtual server.  If preface is not matched,
HTTP/0.9-1.1 is assumed.