MINOR: acme/lua: implement ACME.challenge_ready() Lua function

MINOR: acme/lua: implement ACME.challenge_ready() Lua function

Add a new ACME global Lua table with a challenge_ready(crt, dns) method
that wraps acme_challenge_ready(). It marks the ACME challenge for domain
<dns> in certificate <crt> as ready and returns the number of remaining
challenges, or 0 when all challenges are ready and validation has been
triggered. A Lua error is raised if the certificate or domain is not found.

The ACME table is registered for each lua_State via the new
REGISTER_HLUA_STATE_INIT() mechanism.