Allow the value "UNSET" when removing an key event with dnssec-settime

Description

dnssec-settime prints the string "UNSET" for events where no time/date is set for the event

To unset an already configured event, the user has to specify the values of "never" or "none" on the dnssec-settime commandline

Request

It would be helpful from the user point of view (more symmetric, less confusing) if the value "unset" would also be allowed for dnssec-settime to remove the date/time from an event

Links / references

This request is coming from an BIND 9 DNSSEC training at Linuxhotel, Germany, where I observed users new to DNSSEC tried to remove an wrongly configured event with the value "unset", because for them it was the natural choice without looking into the ARM or man page