kzonesign: support for fake timestamp

doc/man/ksignzone.8in

doc/man_ksignzone.rst

src/knot/ctl/commands.c

src/knot/dnssec/zone-events.c

src/knot/dnssec/zone-events.h

src/knot/events/handlers/dnssec.c

src/knot/events/handlers/load.c

src/knot/events/handlers/refresh.c

src/utils/ksignzone/main.c