Harden dnsmasq syntax checking against signals

[DL6ER]

What does this implement/fix?

Retry to wait for result of dnsmasq syntax test if interrupted by a signal

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Related issue or feature (if applicable): #2149

Pull request in docs with documentation (if applicable): N/A

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Checklist:

• The code change is tested and works locally.
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[DL6ER]

Fix confirmed by OP in #2149

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/invalid-configuration-dnsmasq-syntax-check-ok/74928/2