net: harmonize the time units used for TCP keep-alive on DragonFly and other UNIX's by seconds
Follows up CL 542275
Fixes #65809
Change-Id: Iba01efb4ff0fbb7a67840875322f0338337ebb78
Reviewed-on: https://go-review.googlesource.com/c/go/+/565315 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Bryan Mills <bcmills@google.com>
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