OPENDNSSEC-629: retransfer <zone>

New feature: ods-signer retransfer <zone>
Retransfers the zone with an AXFR from the master, ignoring the current inbound serial.
But if n is 10, then buf[9] should be '\0' and then strncmp would not match.

Fair enough: should be. So it may be better to add additional length checks.