When constructing links we want to read the required host and protocol
information in a dynamic manner from the request for constructing ACME
links such as the directory information. This way, if the server is
running behind a proxy, and we don't know what the exposed URL should
be at runtime, we can construct the required information from the
host, tls and X-Forwarded-Proto fields in the HTTP request.
Inspired by the LetsEncrypt Boulder project (web/relative.go).
Make sure we do not pass domains with asterisk (wildcard) in the middle,
like _acme-challenge.*.example.com to lookupTxt function, but preprocess
domain and remove leading wildcard so we lookup for
_acme-challenge.example.com.
Perform domain normalization for wildcard domains, so we do query
TXT records for _acme-challenge.example.domain instead of
_acme-challenge.*.example.domain when performing DNS-01 challenge. In
this way the behavior is consistent with letsencrypt and records queried
are in sync with the ones that are shown in certbot manual mode.
