Also rename the "result_complete" variable in GetSettingsList() to "done" to be
more consistent with GetSetting().
This change doesn't affect current behavior but could be useful in the future
to support dynamically changing settings at runtime and adding new settings
sources, because it lets high priority sources reset settings back to default
(see test).
By removing a special case for null, this change also helps merge code treat
settings values more like black boxes, and interfere less with settings parsing
and retrieval.
Implement merging of settings from different sources (command line and config
file) separately from parsing code in system.cpp, so it is easier to add new
sources.
Document current inconsistent merging behavior without changing it.
This commit only adds new settings code without using it. The next commit calls
the new code to replace existing code in system.cpp.
Co-authored-by: John Newbery <john@johnnewbery.com>
