* plugin/file: make non-existent file non-fatal

If the zone file being loaded doesn't exist *and* reload is enabled,
just wait the file to pop up in the normal Reload routine.

If reload is set to 0s; we keep this a fatal error on startup. Aslo fix
the ticker in z.Reload(): remove the per second ticks and just use the
reload interval for the ticker.

Brush up the documentation a bit as well.

Fixes: #2951
Signed-off-by: Miek Gieben <miek@miek.nl>

* Stickler and test compile
Signed-off-by: Miek Gieben <miek@miek.nl>

* Remove there too
Signed-off-by: Miek Gieben <miek@miek.nl>

* Cant README test these because zone files dont exist
Signed-off-by: Miek Gieben <miek@miek.nl>