config: introduce "built-in" default configuration settings in default.d
This helps providing a more consistent user experience on all platforms and
with all packaging.
The exact location of default.d depends on how Mercurial is installed and
whether it is 'frozen'. The exact location should never be relevant to users
and is intentionally not explained in details in the documentation. It will
however always be next to the help and templates files.
Note that setting HGRCPATH also disables these defaults. I don't know if that
should be considered a bug or a feature.