Exactly one kind of setting-hostname is done, and that's
entirely independent of writing /etc/hosts. Don't make it
a set of flags, use an enum and a bool.
- point to main Calamares site in the 'part of' headers instead
of to github (this is the "this file is part of Calamares"
opening line for most files).
- remove boilerplate from all source files, CMake modules and completions,
this is the 3-paragraph summary of the GPL-3.0-or-later, which has
a meaning entirely covered by the SPDX tag.
- The configuration for writing the hostname (to /etc/hostname,
to /etc/hosts and possibly to systemd-hostname) is read-only,
because it comes from the config file and won't change after.
