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.
Also adds a grubcfg.conf with the following settings:
* overwrite: If set to true, always creates /etc/default/grub from
scratch even if the file already existed. If set to false, edits the
existing file instead. The default is "false".
* defaults: Default entries to write to /etc/default/grub if it does not
exist yet or if we are overwriting it. Note that in addition,
GRUB_CMDLINE_LINUX_DEFAULT and GRUB_DISTRIBUTOR will always be
written, with automatically detected values. The default in the code
is empty. The shipped grubcfg.conf currently reproduces the default
settings from the Fedora installer Anaconda.
Fixes#128.
