alpn: assemble defaults from roles and allow override

Since new roles may be incompatible with http, add support for
alpn names at the role struct, automatic generation of the
default list of alpn names that servers advertise, and the
ability to override the used alpn names per-vhost and per-
client connection.

This not only lets you modulate visibility or use of h2,
but also enables vhosts that only offer non-http roles,
as well as restricting http role vhosts to only alpn
identifiers related to http roles.