libroot: sethostname() now uses ftruncate().

* Before, it would just overwrite the previous name, leaving extra
  bytes from the previous name (they wouldn't become part of the
  host name, but it just didn't look that nice).
Installer: don't replace system/settings.

* Instead of removing "system" in the target completely, only
  replace all of its subfolders.
* The downside of the current solution is that extra files, and
  directories in "system" will not be removed. Improvements
  welcome.
Fix (hopefully) bootstrap build with HAIKU_NO_DOWNLOADS=1

* With HAIKU_NO_DOWNLOADS=1, the check against existing package files
  in the download folder should only be done in the phase that is
  adding packages to be put onto the resulting target image, not in the 
  phase that is adding the bootstrap packages (as here those packages 
  will be *built*, not downloaded).