These functions will be used in the process of replacing fatal(),
check_result(), and fprintf() calls throughout lib/dns/zoneverify.c with
code that does not call exit().  They are intended for:

  - zoneverify_log_error(): logging problems encountered while
    performing zone verification,

  - zoneverify_print(): printing status messages and reports which are
    only useful in standalone tools.

To make using dns_zone_logv() possible, add a new "zone" argument to
dns_zoneverify_dnssec() that standalone tools are expected to set to
NULL.