Get rid of settings merging code in util/system.cpp repeated 5 places,
inconsistently:
- ArgsManagerHelper::GetArg
- ArgsManagerHelper::GetNetBoolArg
- ArgsManager::GetArgs
- ArgsManager::IsArgNegated
- ArgsManager::GetUnsuitableSectionOnlyArgs
Having settings merging code separated from parsing simplifies parsing somewhat
(for example negated values can simply be represented as false values instead
of partially cleared or emply placeholder lists).
Having settings merge happen one place instead of 5 makes it easier to add new
settings sources and harder to introduce new inconsistencies in the way
settings are merged.
This commit does not change behavior in any way.
7f40528cd5