Teach dnssec-settime to read unset times that it writes

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
pages.

(cherry picked from commit 4c96efac5c3df764dc19c0855b0824d553324b96)