ArgsManager: special handling for -regtest and -testnet

Summary:
Detects and avoid setting network paramter in another network section.

Partial backport of core PR11862
Commits 5e3cbe0 and c25321f

Depends on D2435

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2436