Teach dnssec-settime to read unset times that it writes

When there is no time in a key file, `dnssec-settime` will print
"UNSET", but to unset a time the user must specify "none" or "never".
This change allows "unset" or "UNSET" as well as "none" or "never".
The "UNSET" output remains the same to avoid compatibility problems
with wrapper scripts.

I have also re-synchronized the "Timing Options" sections of the man
69 jobs for !6317 with 3361-dnssec-settime-unset in 43 minutes and 16 seconds (queued for 2 seconds)
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