Pass argsmanager reference to intro instead of relying on global

[maflcko]

Motivation: (copied from bitcoin/bitcoin#19779)

The gArgs global has several issues:

• gArgs is used by each process (bitcoind, bitcoin-qt, bitcoin-wallet, bitcoin-cli, bitcoin-tx, ...), but it is hard to determine which arguments are actually used by each process. For example arguments that have never been registered, but are still used, will always return the fallback value.
• Tests may run several sub-tests, which need different settings. So globals will have to be overwritten, but that is fragile on its own: e.g. Net processing: move ProcessMessage() to PeerLogicValidation bitcoin/bitcoin#19704 (comment) or #19511

The goal is to remove gArgs, but as a first step in that direction this pull will change gArgs to use a passed-in reference instead.

[maflcko]

requested by @ryanofsky in #35 (comment)

[hebasto]

Concept ACK.

[hebasto]

ACK fa1b478, tested on Linux Mint 20 (x86_64) with the -choosedatadir command-line option.

[maflcko]

@ryanofsky Would be good to have your eyes over this as a sanity check 👀

[ryanofsky]

I don't think this change is beneficial in it's current form. Before this change there are 46 references to the gArgs global in src/qt/, and after this change there are 40 references to gArgs. I think if you really want to get rid of the gArgs global in the GUI you could do it in a single PR instead of having gArgs and args variables used different places unpredictably.

Also I don't think there is a necessarily a reason to eliminate the gArgs in the GUI. It's understandable to want to get rid of global variables in node and wallet code to allow more code reuse and help testing, but the GUI already relies on a global QApplication instance, a global event loop, global settings state and probably other global state, so getting rid of gArgs there doesn't seem to accomplish much. I think removing gArgs from node and wallet code and then making gArgs a GUI-only variable would satisfy goals stated in the PR description while keeping GUI code less verbose, and requiring fewer changes.

[maflcko]

Ok, thanks for looking.