rework launch script argument parsing (#832)

This commit...

* replaces the internal `--no-reopen-terminal` arg with a documented `--use-current-shell` arg that works on Linux too;
* replaces the new SKIP_TERMINAL environment variable with the existing SMAPI_NO_TERMINAL one;
* moves argument parsing out of the 'initial setup' section (so it's easier for players to edit if needed);
* simplfies argument parsing (no need to support short opt names or add arguments for the default behavior);
* fixes arguments not parsed after the first unrecognized argument, so `--unknown --no-terminal` would still open a terminal.

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