* list_ref can become LUA_REFNIL, because that's what rawgeti returns
for LUA_NOREF. Defensively guard for this, rather than falling into
the sntp_dolookups loop with nil on the stack.
* set_repeat_mode should not call itself, but should rather always do
what it's going to do and then optionally do the rest if directed.
* sntp_sync should not try to special case the single string argument:
we should be queueing that name for DNS resolution, too. Towards that
end, if we are given a single string, build a table and make that the
list_ref and call off to sntp_dolookups, just like we otherwise do.
FIXES: #2699
7d387dd4d6