Split out key-value parsing in ArgsManager into its own function

Summary:
This will allow us to parse CLI args while under test without
erroring on other args the test framework may be adding.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4496