certctl(8): don't completely nuke $CERTDESTDIR

It's been reported/noted that a well-timed certctl rehash will completely
obliterate $CERTDESTDIR, which may get used by ports or system
administrators. While we can't guarantee the certctl semantics when other
non-certctl-controlled bits live here, we should make some amount of effort
to play nice.

Pruning all existing links, which we'll subsequently rebuild as needed, is
sufficient for our needs. This can still be destructive, but it's perhaps
less likely to cause issues.

I also note that we should probably be pruning /etc/ssl/blacklisted upon
rehash as well.

Reported by: cem's dovecot server
MFC after: 3 days