Change gui code to use gArgs, Params() functions directly instead of
going through interfaces::Node.

Remotely accessing bitcoin-node ArgsManager from bitcoin-gui works fine
in https://github.com/bitcoin/bitcoin/pull/10102, when bitcoin-gui
spawns a new bitcoin-node process and controls its startup, but for
bitcoin-gui to support -ipcconnect option in
https://github.com/bitcoin/bitcoin/pull/19461 and connect to an existing
bitcoin-node process, it needs ability to parse arguments itself before
connecting out.

This change also simplifies
https://github.com/bitcoin/bitcoin/pull/10102 a bit, by making the
bitcoin-gui -> bitcoin-node startup sequence more similar to the
bitcoin-node -> bitcoin-wallet startup sequence where the parent process
parses arguments and passes them to the child process instead of the
parent process using the child process to parse arguments.

ninja all check-all