- Move creation and unlinking of /etc/wall_cmos_clock from the handling

- Move creation and unlinking of /etc/wall_cmos_clock from the handling
  of the initial UTC dialog to install_zoneinfo() so that file gets the
  necessary treatment also when that dialog is skipped via "-s", when
  selecting UTC from the time zone menu or on the command-line instead
  etc.
- Make the initial UTC dialog actually work by giving the relevant files
  the necessary treatment and then exit when choosing "Yes" there instead
  of moving on to the time zone menu regardless.
- Since r301131, /etc/localtime is also installed when selecting UTC in
  interactive configurations (which previously meant only via the time
  zone menu, though). Thus, the code added in r230298 which treats a
  NULL zone file name as UTC and removes /etc/localtime in that case can
  go again.
- Consistently refer to "could not delete" (as chosen by the oldest such
  code in here) when unlink(2) fails instead of a to mixture of "delete"
  and "unlink" in error messages.