DOC: config: document the "http-hardening" section and server/bind options

DOC: config: document the "http-hardening" section and server/bind options

Add section 12.10 describing the "http-hardening" section, the implicit
"untrusted" and "trusted" profiles and the "all" keyword, and document the
"http-hardening" option of "bind" and "server" lines. For now we do'nt
have other examples aside "all" in the section but the goal is that its
usage now starts with a few options.

Also take this opportunity for defining the meaning of "trusted" and
"untrusted" in this context.