ArgsManager: support config file sections
Summary:
This adds support for network sections in the configuration file.
There is no special case management yet.
Partial backport of core PR11862
Commits 95eb66d, 30f9407 and 8a9817d
Depends on D2433
Test Plan:
make check ./test/functional/test_runner.py feature_config_args
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2434