This could have just as easily been a tls directive property in the
Caddyfile, but I figure if these challenges are being disabled, it's
because of port availability or process privileges, both of which would
affect all sites served by this process. The names of the flag are long
but descriptive.
I've never needed this but I hear of quite a few people who say they
need this ability, so here it is.
6bc3e7536e