nghttp, nghttpx, nghttpd, h2load: Support h2-16 in NPN/ALPN

The nghttp2 library itself is still h2-14.  To experiment with the
implementations to require h2-16 to test new features (e.g.,
prioritization), nghttp, nghttpx, nghttpd and h2load now support h2-16
as well as h2-14.  Cleartext HTTP Upgrade is still limited to h2-14
however.