netwatch-notify: work around race condition

This used to crash every now and then with:

> script;error script error: no such item (4)

I guess this is caused by querying the dns cache just exactly when a
record expires. The chance is maximized: The script is started by
scheduler every minute, and the record's ttl is a multiple of a minute.

Let's query records that are not about to expire immediately, and
try again.

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