ArgsManager: keep command line and config file arguments separate

Summary:
This makes it possible to know whether an argument is set from the
command line or the config file.

Behavior change: in case of multiple values for an argument, the
following rules apply:

• From the command line, GetArg returns the *last* value
• From the config file, GetArg return the *first* value

Partial backport of core PR11862 (commit 3673ca3)

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2432