Problem description

Via @nkoranova: DAPS shows confusing messages about no xml:base being present when run on AsciiDoc files. These messages show up despite no way for anyone to add xml:base elements there (unless that is fixed in AsciiDoctor):

> daps -d DC-caasp-admin checklink 
Neither file nor rootid specified, using rootid from DC-file
 No ancestor with xml:base for 'link@id=' found.
 No ancestor with xml:base for 'link@id=' found.
 No ancestor with xml:base for 'link@id=' found.
[...]

Expected behavior

• Since users cannot do anything about that (because AsciiDoctor does not generate xml:base tags), can we just suppress these messages?
• I haven't checked the file names displayed in the output properly but I guess it's also not looking great, so those should probably be fixed somehow while at it.

[Related: #561]