In spite of there being considerable documentation sometimes in the
config file, we go with CC0 because we don't want the notion of
'derived work' of a config file.
The example `settings.conf` is also CC0. Add some docs to
it while we're at it.
- Make *mandatory* optional (and default to false); this
allows shorter lists of entries
- Allow degenerate entries which are just a name
(which have *mandatory* set to false as well).
SEE #992
- With refactored code, introducing new kinds of actions
is very few lines of code. Allow disabling targets
(services was already possible). Allow masking units,
but as a special case require the complete name.
FIXES#975
