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