Some distro's let the user change the hostname at will;
others don't, and yet others don't have systemd to change
the hostname with at all. Check if we **can** change the
hostname (as a non-root user), before setting expectations.
- 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.
