As reported by @satwo on IRC a few days ago, the current mechanism of treating unknown config file options as errors is problematic for options like -rpcclienttimeout which aren't defined for bitcoind. A full solution would be to either make all binaries be aware of each other's options, or to permit config file options that only apply to specific binaries (bitcoind, bitcoin-qt, bitcoin-cli). Both of these seem too invasive to introduce for 0.17. As a compromise, this PR makes it ignores those options, but still warn about it in the log file.
Backport of core PR13799.
Depends on D5553 (it is buggy otherwise).