[refactor] Config handling refactoring in preparation for network-specific sections

Summary:

• Move ChainNameFromCommandLine into ArgsManager and rename to GetChainName
• [tests] Add unit tests for GetChainName
• Separate out ReadConfigStream from ReadConfigFile
• ReadConfigStream: assume the stream is good
• [tests] Add unit tests for ReadConfigStream
• [tests] Check GetChainName works with config entries
• [tests] Add additional unit tests for -nofoo edge cases

This is a backport of Core's PR12878

Depends on D1833

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox, schancel

Reviewed By: #bitcoin_abc, jasonbcox, schancel

Subscribers: schancel, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1834