Keep a user-supplied domain parameter in inn.conf or readers.conf

[Julien-Elie]

The domain parameter in inn.conf currently has a weird behaviour; it may not always be used, depending on a few rules that need being reviewed. An expicitly-set configuration option should override whatever is discovered while probing around on the system.

We should:

• Use domain unconditionally if it is set in readers.conf for forming Message-IDs and Injection-Info.

• Use domain if it is set in inn.conf, regardless of what we can discover from other system calls.

• Document domain as a first-class configuration parameter for access groups in readers.conf, saying that it changes the way Message-IDs, Injection-Info and the newsmaster e-mail in HELP are constructed.

• Update the wording in the FAQ.

[Julien-Elie]

Implemented in INN 2.7.1 except for the second point (using domain from inn.conf which is not backwards-compatible) postponed for the next major release. This will be tracked in #244.

As for the newsmaster e-mail, let's document existing fromhost parameter which is exactly supposed to configure the domain part of e-mail addresses.