MFC: r322097, r322203

• Since r301131 (MFCed to stable/10 in r321895), /etc/localtime is also installed when selecting UTC in interactive configurations. Thus, the code added in r220172 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.