core: force emission of DNS_CONFIG_CHANGED signal on SIGUSR1

Beniamino Galvani requested to merge bg/dns-update-on-sigusr1 into main

If the configuration contains dns=none and resolv.conf is updated through a dispatcher script, currently there is no way to tell NM that the content of resolv.conf changed, so that it can restart a hostname resolution.

Use SIGUSR1 (and SIGHUP) for that.

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