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.

ninja all check
./test/functional/test_runner.py feature_includeconf