dns/dnsmasq: backport fix for leftover child processes

For details, see
https://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commitdiff;h=15841f187d2b208a6113d4e2d479d3af4275bb1c

Reported by: novel@ (Roman Bogorodskiy)
Obtained from: Simon Kelley via upstream Git repo
PR: 287324

dns/dnsmasq-devel already has the fix as of v2.92test14.

Thanks to Roman for the report, upstream communication, and test.

(cherry picked from commit 27a9f282940b43820ba2f4d92e7e96615d814bda)