Convert verifyzone() to a libdns function

In preparation for implementing mirror zones, the verifyzone() function from bin/dnssec/dnssectool.c should first be converted to a libdns routine which does not call exit() under any circumstances. The reworked function will later be employed for verifying mirror zones. dnssec-signzone and dnssec-verifyzone should also use the reworked function to prevent code duplication.