use sendmmsg syscall towards UDP clients

Needs squashing and NEWS, but otherwise probably complete.

With a simple single-threaded benchmark I'm getting ~11% improvement in QPS. (In real life the impact should be much lower.)

yes 'spackovi.net A' | dnsperf 127.0.0.1 -p 5353

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• 13eb8ab4 - fixup! meson: polish sendmmsg configuration

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marked as a Work In Progress from 13eb8ab4

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• 18d7235f - fixup! meson: polish sendmmsg configuration

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• 2f74829e - main: better support for profiling
• cce47d3c - daemon: use sendmmsg towards UDP clients

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• e660c2d4 - daemon: use sendmmsg towards UDP clients

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I expect NEWS can wait. There should be more similar changes soon, and details won't typically be interesting, so we can write some summary description later.

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• c99d8e6b - daemon: use sendmmsg towards UDP clients

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merged

mentioned in commit d161193b